green star polyps, mushrooms, ricordia, and zoa.
They will all be the same difficulty.
I'd go with some expensive ricordia Yuma since you can easily cut one into 8 pieces and watch them all grow out. In a 2g tank you'd have low flow and low light, all conducive to the healing process of the shroom. In about 3 months you'll have a field of rics. They then tend to reproduce like crazy when several of them are clustered together.