Sterbai Corys can handle low 80s with no problem, and they're pretty easy to find. I've heard of folks using them with discus at 86 F, though I don't know if that is conducive to their normal longevity.
You can do lower-temp corys in the low 80s, but their metabolism will be increased and their lives will be a little shorter.
If you're going to keep your Peppered Corys, I'd not do BLUE Rams. However, Bolivian Rams would work fine in the mid-upper 70s.
If you want to do 10 corys in your 20 long and want to keep the Peppered, I really think that either a single Bolivian Ram or a single male Cockatoo Apisto (A. cacatuoides) would be your best bet for a centerpiece fish. These fish will thrive in the same temp range as your corys AND they're *much* hardier than Blue Rams. (You *might* be able to do a pair of A. cacatouides in that tank. I've never kept them, so I'll let those with experience chime in. Bolivian Ram pairs tend to be more aggressive towards tankmates when breeding, so I'd avoid those.)
As has been mentioned before, Blue Rams are very fragile and are not a fish for novices. I have near-optimal water parameters (pH 6.6, 3 dGH, 5-10 ppm nitrates, low 80s F, three water changes per week) and wouldn't be surprised if mine kicked the bucket six months from now. The typical stock at the local LFS probably won't last more than a few months in a well-kept aquarium. Advanced aquarists who purchase healthy and genetically-tough stock from a breeder and keep their tank in tip-top shape can expect anywhere from 1-2 years out of them, maybe 3 if they're lucky.
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