Your fronts that are pictured above are small still. Once they hit 10 inches you will understand what a spooked front will do. I have had them push glass tops up with 5 pavers holding them on.
The livingstonii will quickly out grow the calvus and are pretty aggressive when the biggest in the tank.
When thinking of your tank long term, you will want no more than 7 fronts in that size tank, and perhaps a few comps or calvus for tankmates. I would get larger comps then fronts. The fronts do grow slow, but they will quickly out grow the comps.
This information comes from my experience with all of these fish in multiple tank setups. This advice also comes from the countless discussions and threads where people have had the same questions.
