I just put a 20 gallon high online as a frag tank, this is tied into my 60 gallon cube and sump. Should i add a few small fish and a small clean up crew or am i asking for problems?
As long as it's inline with your system, you shouldn't need to unless you develope algae problems imo. I some times like to use frag tanks for fish that can't go into my main display for what ever reason.
I would add a melanarus wrasse max to control the regular annoying small brown planaria ( that is if you have any ). Other than that it would be more for your viewing preference.
I had livestock in mine, just be careful not to add inverts that will tip over your frags, I was surprised how easy it was for them to dislodge plugs placed in my frag rack.