If your going to setup a little nano tank and you want a simple coral. I would get some xenias. They are a soft coral and very pretty. They are easy to take care off.
As for the sump its really up to you. I say the bigger of a sump the better and easier to keep your water in check.
As for the sump its really up to you. I say the bigger of a sump the better and easier to keep your water in check.