I do agree that the Red tail cat is a big concern considering their size and the possibility of eventually eating any smaller fish. They are tank busters (a quick Google search will show you their potential size), and not really suitable for most aquarium sizes in the hobby...I think it would be better to start looking for a new home for the RTC rather than others, since it will need to happen eventually, unless you're thinking about a much bigger upgrade down the road (i.e. a good-sized pond).
If you're having trouble finding a suitable owner, I would strongly suggest asking at one of our sister sites,
http://www.monsterfishkeepers.com/
The people there are dedicated to keeping LARGE fish, although to be honest, I'd assume that even the majority of the people there do not have the means to keep an RTC. BUT, they likely know of better ways to go about the situation than I would.
In terms of stocking the rest of your tank, I'm not the best when it comes to semi-aggresives, so someone else would give you better advice there...but I will ask, do you happen to know what species the large/small "algae eaters" are? My guess would be a type of "common" pleco, or maybe a Chinese algae eater (CAE), but if you could post pictures it would be helpful moving forward with your stocking questions.