SRAM makes shifters rear derailleurs with 2:1 actuation, and 1:1 actuation. The former will work with Shimano, the latter won't.
If you use a 9 speed front derailleur with a 7 or 8 speed cassette, regardless of the shifter used, your chain will rub at some point. The 8 speed (or 7) chain is only fractionally wider than the 9, but just enough that it will cause problems on some applications.
An 8/9 speed hub will accept a 7 speed cassette with a small spacer.
If you must keep the thumbies, go with XT 8 speed from ebay, and you'll have the easiest time finding parts. If that's not within budget, I can only suggest a unicycle.