yea, should be ok, a little too small though for anything stable, get a bigger tank, above 50g and use the 20g as a sump, it will save u money in the long run, becuase things will die easier etc, it will also be easier
If I can make room for one i would like to have a bigger tank. I got a live rock question also, instead of buying like 100 lbs of live rock could i buy a bunch of base rock or make my own rocks like on GARF.org and then seed it with some live rock?
I think a 20 would be OK, especially if you are using a sump. The main problem is that it will limit what you can keep. Only a few fish, and you will end up pruning corals a lot to keep them in line.
You can make your own rock, but it can be a hassle. Depends on what your time is worth. Might be better to buy base rock and seed it.