I used a 300 gal stock tank as an "Indoor Cichlid Pond" for years and was very happy with it. I'll admit the top view only was restrictive compared to a typical glass/acrylic aquarium, but the solid side walls seemed to make the fish feel more comfortable.
I found I could fit a whole lot of fish in it, although I was keeping 6~12" Cichlids and not 30" catfish. I think something this size would be okay (but not 'good') for one or two 30" cats, but not 5. The diameter of the tank is only a little over double the fishes length. It is quite true that they could swim in circles thus allowing them a much longer "linear path" but 5 large cats with 5 large hides they will be constantly running into one another which will likely interrupt their "linear path".
I have also seen some 8' round 700 gallon kiddie pools that I was very tempted to buy. I think the little bit of extra diameter would make a huge difference in your case.
As for filtration, I highly suggest you consider a Pressurized Pond Filter such as the BioForce 2000. I used this filter on my stock tank for years and was completely satisfied with it. It works well with up to an 1800 gph pump.