I got a few crowned pearscales, and noticed today they were starting to chase a little, along with a few of the others in the big comunity tank. I started to set up a twentygallon ( wanting to just breed the pearls togeather) and right now, i have it cycleing with just a back filter with a sponge over the outtake thingy (Technical arent i?). I read somewhere that there shouldnt be an underground filter in the tank you plan to have the eggs in. would just the back filter work? or should i set up the undergravle and put lots of gravle in along with some kind of plant for them to stick to?

A sponge filter is really the best for eggs and fry, its very gentle, doesn't cause much current or sucks up the fry. A sponge filter is a simple thing, and you can get a few variations for 8 bucks at the pet store. Its run by an airpump and connected to airline tubing, thats it....
A 20 gl sounds perfect for spawning and eggs afterwards. I would add some floating plants, Anacharis would be just fine for that. When I found eggs in one of the ponds, they were all sticking to the roots of some water hyacinth.
Good luck with your pearlscales! I just those!