So my specific question is, which victory should I try for?
You said that you were focusing on tech and tourism, if you think a you can land a tourism victory, then go for it. Otherwise scientific is a nice fallback. You're going to be focusing on tech to keep up your tourism anyways, so there really isn't any harm in it.
Keep in mind that victory-conditions isn't exactly set it stone, you can severely increase your chances to win a tourism-victory by eliminating a high culture rival/steal wonders/steal great works or by getting a few extra city-state allies and passing some favorable world congress resolutions.
I'm not saying that focusing on one particular victory-type in CPP is bad by any means, but the game is far more open in that regard compared to vanilla where you pretty much had to decide from the start what to go for.
A more general question is, do the different victory conditions lend themselves to either tall or wide strategies, or can they be pursued either way?
First of all, with the way CPP works, you really don't talk about Wide vs Tall anymore, you pretty much always want your cities to grow, no matter how many cities you settle, more population means the ability to work more tiles and more specialists, resulting in more gold, more science, more culture and more more production. The only real drawback of growing your cities is that happiness becomes slightly harder to manage, this however shouldn't really be a problem.
From my experience getting more cities never actually hurt. Depending on the game-situation it could take a while for them to actually be a boon to your economy instead of a drain but more cities pretty much mean more of everything. The biggest drawback to expanding, either by settling or by conquest is that it tend to create more enemies, which makes staying peaceful a lot harder.
As for specific victory-condition, in my experience Domination victory is probably the VC where it is most clear that bigger is better. More cities means you can produce more units and it means you can gather more strategic resources. As a byproduct of going for Domination you're probably going to get a bigger empire anyways, either because you need to conquer land to make your way towards the other civ capitals or just because you don't want potential enemies settling cities too close to you.
For scientific VC, more cities usually means more science, more cities also increases the cost of techs by a bit, but unless you're really overdoing it, or completely neglecting infrastructure in your cities, the science from your new cities are going to more than compensate for that. More cities also means the ability to work more scientists, meaning you'll end up with more great scientist to consume.
However, it is worth mentioning, that a Wide empire and a Compact empire plays the science game slightly different. If your empire is Compact you're going to benefit a lot more from science from trade-routes or science from academies. Trade-routes and academies are going to provide you with a fairly significant part of your science income if you're playing a smaller more compact empire, while if you're playing really wide the flat science gained from those sources are fairly negligible.
For Tourism victory, if your empire is bigger than your enemy's he is going to get a boost, both defensively and offensively for tourism towards you. This bonus however is usually not enough to compensate for the sheer amount of extra tourism and culture you can get by expanding but it is probably still worth mentioning.
Compact Tourism play has a few benefits however, tourism over your enemies means your trade-routes generates extra flat science along with extra growth and extra gold, this is a lot more valuable if you have a low number of cities for reasons already explained earlier.
It is also worth mentioning that Compact Tourism and Wide Tourism plays slightly different, with Compact putting a lot more value towards wonders, great works and themingbonuses, as they are flat increases. Wide tourism instead are going to earn a whole lot more tourism from historical events, as your total culture-output is a lot bigger, they are also going to trigger more historic events by birthing more great people and also gathering more faith that can be used to buy extra great people.
Diplomatic victory condition like domination also heavily favors wide game-play. More cities means more paper which in turn means more diplomats. More cities means you're in bigger control of the votes you get from religious authority, it also means you're going to generate more great diplomats, which can be used for even more votes. I'm honestly not sure there is any specific benefit to staying compact if you're going for a diplomatic victory.
Hope this helps you out.
