Ok well those cories in the 6g should probably go in the 15g when you get it/have it set up
No one is trying to be mean to you or mess up your plans. Those fish truly thrive and will display their natural behaviors to you (which I'm sure you'd love to see!) when they are in a larger group in an appropriately-sized tank. It's nothing against you personally that people are saying this. It's because it is true and it is ideally the conditions that those fish should be in.
I do agree that the 6g is too small. For the moment, it is what it is. Please make sure you do water changes I would say every other day or every three days at MOST, about 50% water change. The tank is uncycled, correct?
We had our female betta and the 7 pygmy catfish in a 5g and that was too small for all of them. It did LOOK like they all had plenty of room but really, no, plus when you factor in waste production and such, it just was too small. We only ever did that with the intention of it being temporary of a few weeks to a month while we finished readying our 10g tank for them.
I have done some good research on catfish in general, but specifically pygmys (since that is the only kind of catfish that I found should be in a 10g tank, especially when you consider the number of them that you should have at minimum), and catfish really do need to be in larger groups. The folks on here confirm what I already knew about that. They will be happier, shoal, and show you how much fun they are having when they are together in ideal groups and in an appropriately sized tank.
The other issue with the catfish you have in the 6g tank is waste production. It could quickly get toxic in there if you forgot to change the water often enough. Do you have any testing kits?