if this is your 180ltr tank (listed in your profile)
I would say that you are at your limit of current stocks.
If it were me, I'd rehome the clown loaches, pengatius, congo tetras and sail fin plec
then I'd add at least 2 more tb's.
once that is done then, and only then, would I consider adding more fish.
I wouldn't go for serpaes but somthing similar like the Roberti Tetra Moenkhausia robertsi
Yeah it is, ive gone off fish per gallon (bit under actually) but only because my plec and pengatius are only small babies! im getting a 400l tank in about 3months (maybe a little smaller) to replace the 180L and rehome all the fish.
I'll heed your advice and stick with the gang ive got! They are great!
but i do know that my bigger fish will need a bigger home soonish!