corydoras paleatus

i have 3 Corydoras (left from an old tank that didnt do well, and was constantly plauged with disease (even though the water parameters were goof and they were getting regular WC, nothing extreme, only problem was some "painted" gravel, which had chipped exposing some porous material underneath))

however they are currently residing in my 50g SA tank, which contains geophagus and mikro geophagus, and obviously the temprature is high for those fish (28c) given that this sp prefer cooler temps i'm wondering if its cruel to keep them there (they've been living in similar temps for the last 3 years +)

however the only other tank i have that they can go into is a 12" ube, which has no heater whatso ever, and is densly planted and contains shrimp curently (and no filter @ the mo) and i dont think its big enough for them. esp as there swiming space is somewhat restricted by the plant growth.

I would like to add some C. sterbei (who would cope with the high temps) to the main tank, but from what i've read elsewhere different corydoras sp genrally dont mix with each other, so boosting the numbers of corydoras that way wouldnt work.

what would you do (bear in mind i'm sort of banned from getting more tanks, i've been told that 12 is too many)