I agree that the Shoshone are a very strong civ. I hesitate to call them "OP", because there are a lot of strong civs in the game, and overall game balance is very good, even if there are distinct tiers of strength.
I'd also agree that they CAN win any victory type, and their strength helps towards this. So in answer to:
"1. Which victory conditions are they suited for?"
The answer is indeed, all of them.
However the victory type they are BEST suited for is Domination. Reasoning is as follows:
1) Boosted early borders means aggressive landgrabbing and earlier than normal border tensions with neighbours. This is a small factor, but has a tendency towards earlier wars.
2) They have two UUs and no UB. This is a small factor, as neither of their UUs wow in warfare as they stand. Still, any civ that has two UUs is normally one that is best served going to war (with the possible exception of those with non-aggressive GP UUs, like Venice).
3) Pathfinders upgrade to Composite Bowmen. This is the big factor. Pathfinders are too strong to not immediately have 2 or 3 of them. Controlled ruins means you can get CBs earlier than anyone in the game. This means you utterly dominate early warfare. Not using this in a war is a great way to waste your potential. You can't quite do a 6-CB rush without CB tech (well, i'm sure its possible, but its not something you'll usually attempt) but two or three CBs when everyone else is on tiny cities and warriors and maybe archers will easily win you a capital or two.
Once you've started early warmongering, the easiest path to progress to is Domination, though to be honest it sets you up well for most victory types. A relative lack of late game advantages means as the Shoshone you want early warmongering rather than late warmongering to be your decisive game phase.
In answer to how you use comanche raiders, I'd say "as an afterthought". Don't bother beelining them, as they're not game changing, but when you get them they're a nice minor bonus from the baseline.