You will want to QT all the fish and treat with either hypo or copper. Also leave your display tank fallow for 6 weeks so that what is left in there can die off. It can, and likely will, come back otherwise. Read up in marine Ich so you totally understand the enemy and why all of this is really important to irradiate the parasite, not just "treat" the fish.
A 20g is really going to limit you. 75g is where you start having better selection options, IMO
. I've got a 75 and already wish I had gone 120 or 180g.
Sites like liveaquaria.com have great info on tank requirements, compatibility, etc. for future fish purchases, don't hesitate to ask here. You will get solid advice, rather than advice from a LFS
that often give questionable advice to make a buck.