ah, see
that's what you should have said to start with
most commonly available cories have an adult size of about 3 inches and of course, the more the merrier for these little schooling fish. no fewer than 4 and probably no more than 10 if you want to have something else besides cory cats!
remember, cory cats need a very fine substrate or else they will wear off their barbels and die of starvation. most cory cats have a natural lifespan of ten years or more, but that does vary across the species.
i actually seem to remember reading that bronze cories (
C. aneaus) do fine in temperatures as low as 75 F, but don't hold me to that.
if you really want a coldwater bottom-dweller that exhibits a lot of the same behavior as a cory, you should consider a small group of dojo loaches. HOWEVER, dojos get quite large (a foot long!) so you could only have only a couple with
no more than 3 goldies. but that could in turn make for a beautiful tank...
you should double-check this recommendation; i'm by no measure a coldwater expert.