Platies are typical hard water fish: they will love your conditions. I keep mainly livebearers for this specific reason; they do fine.
Fish that prefer hard alkaline include: platies, mollies, guppies, swordtails (and other relatives, such as limias, heterandrias etc) and many types of rainbow fish; also Lake Inle fish like galaxy rasboras and emerald rasboras. Also East African cichlids (though they are not for a community tank like the one you are planning).
Fish that don't mind include: bristlenose plecs (mine have bred prolifically in PH 8.4), many species of corydoras (again my peppered cories spawned frequently), many danios, a few tetras (black neons and black phantoms should be fine).
Oh, and shrimps like hard and alkaline as well.