The recommended fish tank size for a fish is the total area the fish has to swim around, not total volume including the sump. Think about it... if you have a 10 gallon tank and a 60 gallon sump, could you house a fish that grew to a foot long?
The added volume is to help stabilize the water conditions, temperature, and overall increase good bacteria. Not to mention sumps are GREAT to house all of your equipment. Go for fish that are recommended for 50 gal
or less, unless you're planning to upgrade VERY soon before the fish get too big. HTH