I think the P. Elongatus may work well. The blue johanni may work well also. I am going to add a group of one or the other.Yes, added a group of P. Elongatus the other week to my setup, and hasn't upset the balance at all. There is an elongatus that is much more aggressive than the others, but I seem to have landed ok with the more docile ones.
A male has emerged from the 3, which is quite stunning in his full colours. Quite amazing for £3 each - no-one seems to want them in this country!