Nobody is short enough for the 24"...except Bairdco.
Just kidding Baird =)
I think the biggest reason people use 26" is because without messing with gearing you'll get more top speed out of one (1 revolution of a 26" goes just a bit further than a 24").
As for gastanks, you can check your local motorcycle salvage yard and maybe come up with something. Or do some fiberglass work on a stock tank. Or just weld up your own.
I did some fiberglass work on my stock tank and it came out like this.