If the 20g is cycled, I would suggest four more corydoras so the two you have now feel more comfortable and happy (or at the very least, on more).
Those guppy fry are still pretty small, right? You could transfer some for now. I would think they're too small yet to cause much of a difference. But that's a lot of gupster/platy fry. Once they're full grown, I'm not sure there is room between those two tanks. Do you have plans for upgrading?
Also, HAPPY (soon to be) BIRTHDAY!!!!!!!! Yay for fishies on birthdays!!!!!!!
