I have played well over 25 complete games, and I almost always started as the English or French, but I have recently played a couple of games as the Spanish.
Lately I have been holding myself true to my starting nation. You are issued a Letter of Marquis by your starting nation, upon arrival at the first city. In actual pirate days, a letter of marquis only allowed you to attack their enemies or pirates that acted against them. You were supposed to keep a detailed log of who you attacked, how it happened, and what you took. Then you would return the ships and the items to the nation that issued the letter. There would be an official proceeding to determine if the captures were legit. If they were, you then got to keep what you took (usually minus a healthy cut for the crown). If they weren't legit, at best you didn't get to keep the stuff, and at worst you could be charged with piracy yourself and hanged.
So, I will only attack the shipping and cities of other nations if they are at war with my starting nation. If those attacks also favor some other nation, I don't mind reaping the benefits from them, but if they then go to war with my starting nation, I don't care about the promotions, they are now the enemy. I never attack the shipping or cities of my starting nation, with the exception of the Baron, Marquis, or Colonel, if I am playing as the Spanish.
Playing as the Spanish can be a challenge starting out. The other nations don't have as many ships to attack, and it doesn't seem that their ships carry as much gold or goods. So, it is difficult to keep enough gold to keep the crew happy early in the game, but you also have many more friendly ports, and many more daughters to dance with.
There is a way to make big money playing as the Spanish, and it is even easier to do if you play in the early eras. I think this is poor game design, and it's probably an exploit and should be fixed in a patch, as I can't possibly think of a reason why they would make the game like this, but if all the cities and towns (not settlements)of a nation are eliminated, then that nation's ships will start hauling huge amounts of gold. If they are at war with the nation you are representing, then they will send 2 ships to intercept any escort mission. You can kill those two ships and get mega gold.
In my current game, I am playing as the Spanish in the 1600 era. I took out the English in my first year. I was then sailing to Tortuga to take out the one French town, but Spain beat me there and took out the French. That left only the Dutch with the one city of St. Martin. I took that out a couple of years later. So, within 3 years all the other nations were gone. I got Spanish advancements for sacking the other nations towns, killing the named pirates, and for doing some escort missions. After I finished the quests, I just do escort missions. I start over near where Trinidad usually is (it's not there in 1600). I stop at every Spanish settlement and Jusuit Mission, and do whatever escort misssion they will have me do. A pair of French, English, or Dutch ships will usually intercept the mission. About one in 5 or 6 times, it will be pirate ships. The pirate ships don't have much gold on them, but I generally average about 30,000 gold (I have gotten over 30,000 gold from a single ship many times) for the two ships of one of the enemy nations. I sail from that area near where Trinidad is supposed to be to Cartagena. There are about 5 Spanish Settlements and 6 Jusuit Missions in that area. I do those missions, and then sail back east and start over again. I have collected nearly 1,800,000 gold, and my pirate is only 34 years old. I will post some exact numbers tonight or tomorrow on how long each run is and how much gold I make from the run. I think it will be possible to retire at 45, having plundered over 3 million gold.