A nice shoaling fish get on well with guppys like, cardinals,harlequins,black neons, be careful when choosing fish with guppys as some fish like to nip at there lovely flowing tails, a nice shoal of 8 to 10 would be nice.
Get a shoal of harlequins as they shoal most of the time, where the other's stop shoaling once they feel safe in the tank, and a large group of harlequins looks amazing.
is your tank cycled?
i would definitley wait a few weeks and watch your water parameters to see if anything spikes, then add some more fishies. i don't have any but harlequins sound great! what about some corys for bottom feeders?