Most any of the other mild 'community fish' will be fine with the ones you already have. Other livebearers (mollies, swordtails), danios, rasboras or small tetras. I like to put danios in with livebearers in case they have babies because livebearers are like the rabbits of the fishtank and will fill it up with baby fish in no time...and danios are really fast and have a good appetite. Also, do you know what sex your platys and guppys are? They do best when you keep all males or all females...or in groups with at least two females for every male, so you might consider rounding out those numbers if you have one male and one female of each or something like that.