Looking at the pics and taking into account the "invisible tankmates", I'd say the OP could add another couple of fish, but you just can't say for deffinate as s/he hasn't posted the tank size The possible caurse of the Cyno may be high nitrate/phosphate though, and adding more fish would just make that situation worse
Hard to say realy Simonas, but there probibly is scope for more stock
All the best
Rabbut