The only potential challenge to having all curves is getting the train on the track. Its a little easier to do if you put a rerailer on some straight track. Other then that I think you will be fine for what you are trying to do.
Chris
Its not HO, but last year this was my set up. 42 in. diameter circle on the outer loop. In HO the 22" radius circle should work well. Good luck and have fun with it.