Basically, no, your tank is not big enough for all those fish. 75 gallons is big enough for one Oscar and a tank mate. Like I've said already, tinfoil baths can grow to around 12 inches, they are very messy indeed unable Iti out of house and home. They'll put a huge strain on your biological filter. I had two of them up until recently, I got rid of them because they were just too messy. I had actually rescued them so I hadn't actually planned on getting them in the first place. I would never ever get any more. Jack Dempsey's can reach around 10 inches so with that and your Oscar, you still might be slightly overstocked. You will just have to keep a very close eye on your water quality. Make sure you have got a very good filtration, that is extremely important, one of the most important factors in keeping Oscars.
