The possibilities are endless as you have a rather large tank keeping small shoaling fish. You seem to have good numbers in the shoals you currently have. I personally wouldnt do guppies, as I have had the experience of guppies from hell (they were like the mafia!!) What about a shoal of 10 Harlequin Rasboras? Or even chile rasboras? chiles are quite colorful, they are small and look great in a group. I think they would suit what you have a lot more than some guppies. Harlequins may not be the most colorful fish, but they have a lot of personality in a shoal.