We temporarily had our 7 (now 6, we lost one sadly, after he/she was in the 10, we had him about a month) pygmy catfish in a 5g tank for a couple weeks and for about 5 days our female betta was in there with them. We changed the water every 2/3 days.
We knew that having them in a 5g was inappropriate, and it was only done for a short period of time. (Checking on catfish health and then when adding the betta, we didn't have another tank for her with a heater and such, while we tested the cycle on our 10g). They are all now in the 10g.
I want to add a couple more fish to my 10g, but multiple people on here have told me that is not a good idea. Yeah, I think that stinks, but, I care about my fish. I don't want them to be stressed or not have enough room. If we add anything, maybe 1-2 more pygmy catfish or maybe some cherry shrimp (I have to research that option some more) but that is it. Our tank is fully cycled and I could probably add a couple fish and the bio-load would be fine, but, they wouldn't necessarily have enough room, especially if our female betta tried to chase them.
I don't think it's wrong to compare a human in a tiny space to a fish in a tiny space. Or a dog in a tiny space to a fish in a tiny space. We are all living creatures with basic needs in order to thrive.
Could you at least get a 10g? Those extra gallons would at least provide some extra space. A 15-20 would be ideal though. If you went up to the 20, then you could get some more friends for your current cories and they'd be so much happier
I know fish keeping can get expensive. I had no idea how much things could add up. We spent about $200 on our male betta to start... worth every penny. We've spent another $100-200 more than that since then... we really enjoy our fish though and want them to be happy, so, it is worth it.