apart from 5.5gallon being IMO too small, I have 5 Trill's in a 15gallon.
if you want to start within a few weeks, some times even days, buy the biggest ones about 1.5" - 2" as these will be mature (in most cories)
Spend a long time watching them make sure you get fit, and healthy fish, sometimes males can look like females at first glance, so watching them helps you to identify, when I bought my trills I spent a good 30mins-45mins looking at them.
and I got the 3 females and two males I wanted.
You will need some place for them to hide out, such as a cave or large-oish piece of monapi or bogwood.
Cories can be quite skitish in bare tanks so try for plastic plants so you can remove them and add them to the fry tanks, as they will lay on them.
hope this helps.