I know that you can't use OBSE with steam, or that you have to use a backdoor way or something. But several popular mods like Integration require SI so i'm looking for a way to get it, and i'm thinking of just getting it through steam.

Steam and OBSE work completely fine together, and the way to do it isn't difficult or a backdoor way either. Just different than using OBSE with disc version. So all mods that work with disc version works with Steam version as well.

I most likely know the answer here, but if i want to install SI, i'm going to have to reload all of my Oblivion mods right?

Pretty sure all you'll have to do is rebuild your Bashed Patch, if you're using that. I mean, you might also want to install SI-specific parts of mods that have them... but the bulk of your list shouldn't have to change at all, and I don't think it wipes your load order or anything.

Nope. You should be able to add Shivering Isles no problem. I'm assuming your situation is something like this: You own Oblivion, and you want a lot of mods that require SI, but you never got SI. You COULD just get GOTY from Steam, or a store (most stores still carry it, like Target and whatnot) or you can just buy SI standalone from http://www.amazon.com/Elder-Scrolls-IV-Shivering-Isles-Pc/dp/B000NGRIDI/ref=sr_1_1?ie=UTF8&qid=1319420087&sr=8-1 and install it. Then you don't have to worry about anything at all, really.