Cyclaholic, are you going to raise the second rear triangle any? If you don't the frame geometry will be off and you'll probably need shorter cranks. I have the same sized wheels and I had to raise mine to get the top tube back level. I don't think the 24'' rear wheel is a good idea since its normally a good idea to get as much BB height as you safely can for these extended bikes. Whenever I had the bike mocked up, I used 26'' wheels [frame meant for 27''] with 170mm cranks [165mm now], and that was enough to give me some problems with striking.

EDIT: One solution to your seatstay problem could be somehow using an axle that goes through your bottom bracket to mount the second rear triangle. You can probably find some large washers that fits right in the cups or something like that.