If I understand your position correctly, you're asking if Stalin would not have risen, or tried to rise, through the ranks of another political organization were he able to do so, and simply used the communist party as his tool of choice to get to power, because it was there?
If that is your proposition, then no, I don't agree with it. It seems clear from a very early point, that Djugashvilli was moved to join the Bolshevik faction out of political conviction. He worked for many years in a rather obscure part of the country, doing local organizing and agitation, long before he was recognized by Lenin, or brought into a higher fold of the party. He was not always a leader, but he rose to become one because of his efficacy. He had a penchant for getting things done, and quickly, and Lenin particularly liked him because of his very impoverished origins (although we obviously know that Lenin was not a chauvinist about this quality in people) and in particular, his conviction to the communist cause and to Bolshevism. So unless he was willing to dedicate years of his life to obscure low-level party work as part of his planned meteoric rise to power, and was able to hide from Lenin his non-communist political beliefs, then he must have, at least until the death of Lenin, been a communist.
A better question would be: was Stalin a good communist? This is a more apt question because, having determined that he was in fact a communist and not simply an opportunist who latched onto whatever would give him power, we can then examine just how closely his behavior, ideas, and policies resembled both communist theory as well as the political realities of Sovnarkom/USSR once it came into being.
It is my opinion that he was not. A communist is internationalist, anti-chauvinist, and progressive. Stalin was nationalist, chauvinist, and conservative.
This is not entirely his fault. The Russian Empire was a large agglomeration of nationalities who could not be expected to simply give up their ethnic identities (or at least the political identities tied to and defined by that ethnic identity) in favor of greater human unity, such as communists would generally prefer. This pragmatism is best expressed in Marxism and the National Question, which I don't regard as being particularly Marxist, but nonetheless useful for the Bolsheviks in coming up with a coherent policy regarding the several dozen ethnicities under their rule. Had the USSR and Stalin come to be in the US or Great Britain instead of Eastern Europe, where the ethnic national problem is not so acute, then he probably would have never written that book, and probably not needed to.
He was chauvinist not merely in the classic sense, that is, in his dealings with and regards for The Fairer Sex, but also in his attitude toward other countries and ethnicities. Stalin clearly thought of himself as better than women - he abused both his wives, and was very crude when he disagreed with other women like Nadezhda Krupskaya, Alexandra Kollontai, and later Ana Pauker - but he also saw Russia, and Great Russians, as being at the top of the socialist totem pole. It was Russia who had conquered Capital, and was leading the way forward to socialism, and all must follow Russia's lead. And within Russia, it was the Russian language which took precedent over all others, and ethnic Russians who benefited the most from his policies. During the Leninist period, the attitude was one of brotherly leadership, that the Russians were the most developed of the peoples in the former Empire, and so they had a duty to help the more backwards areas and peoples, like the Uzbeks and Turkmen, for example, to come up to speed with where they were. It was not intended to be a lasting hierarchy, but rather the extending of a hand downward to lift the others up. This part of Stalin's personality is best manifested in his dealings with Eastern and Central Europe post-war, where the idea of separate paths to socialism was almost fully disregarded, as much as the Soviets could get away with doing. However, one can see this policy at work also in the dealings with communist parties throughout the interwar period. The Twenty-One Conditions were created to allow a common ground of understanding for the coordination of communist parties in what was then still believed to be an immediately imminent world revolution. This was the guiding principle of the Third International, and perhaps understandably so. But Stalin took it a step further, with the so-called Third Period, when moderate leftists became the principal enemy to be combated. The Twenty-One Conditions never disallowed the existence of other leftist parties who did not adhere to the Conditions, it simply did not allow them membership in the Third International. But Stalin turned this on its head and disallowed cooperation with any leftist party not a member of the International; indeed, slandering them as non-communists and "social fascists." To him, the only communists were ones who took their direction and emulation from Moscow, from the Russians, who had crafted the only correct way to fight capitalism as well as to build socialism. This is chauvinism at its finest. And it is not in line with the attitudes of most other Bolsheviks, including Lenin, who defended the Russian Revolution precisely on the principle that Stalin decried: the Russian situation was unique, and the Russian (here meaning residents of the Russian Empire) proletariat would find its own way to socialism, such that Germans like Luxemburg and Kautsky could not rightly criticize their path, for they failed to understand the necessities and realities of the Russian Situation.
And finally, he was conservative, because of his retrograde social policies. This goes hand-in-hand with the above about chauvinism, but extends to other things as well, like the resurgence of the family unit, an attitude which was later mirrored in the 1950s United States, which held up high the atomic family as the foundation upon which the whole of society must be constructed. It should be apparent to any reader of Marxist literature that the family is a consequence of private property, and that its dissolution was as much sought by socialists as it was concurrently caused by capitalist social relations already. In the first decade of communist rule in Russia, this was the trend. Even if it wasn't very widespread, the foundational thought for the movement was being laid, with Kollontai as its standard-bearer. But with Kollontai's fall from grace and shipment overseas (she was given lots of diplomatic posts like ambassadorships, such that she became unable to exert any influence in the Zhenotdel (Women's Department) in the USSR. Stalin's statements about the highest importance of the family came in the early 1930s, when the USSR was deep into the process of industrializing itself, that is, rushing itself through the capitalist stage of capital accumulation, and while I cannot speculate on the causes of that retrograde action, I can nonetheless point to its contrary nature to what came before, and what communist thought on the subject was. Similar retrograde social polices came through, including a banning on abortions and birth-control and a near-banning on divorces, although the former can be explained by the great need for manpower in the Soviet Union, again a symptom of pragmatism rather than conservatism, but nonetheless a conservative force. The relationship between woman and her rights exists independent of its causes.
So to conclude, I think it is apparent that Stalin was a communist and not simply an opportunist, and that throughout his career he was guided, although very often in a jaundiced way and to varying degrees, by those principles. However, this jaundice does not make him a particularly good communist, either. And as indicated, this may not be entirely his fault. We are, after all, all products of our environments, and Stalin throughout his life showed a loyalty to many of the Georgian social norms he would have been raised around, which can in some ways explain parts of the above, although certainly not all.
