Thanks for the info. It's not a big drop in temp. I was afraid of shocking them. It's usually only a degree, 2 max. Wow, I had no idea a temp drop of 10 degrees would be okay for the cories.
I wasn't sure about the different species thing. Those two are always together and are "touchy feely" LOL They frequently act like what I've seen on the spawning videos, but maybe they're just playing and it's my wishful thinking.
I had the small tank without gravel since it's a nursery tank and I'm feeding frequently and want to be able to really clean the bottom well, especially since it's such a small tank. Didn't know if the sparseness would put the cories off breeding.
But it looks like that's a ways off for now anyway.
Thanks again for all your info. Much appreciated.