1.)They do get pretty big and being from a river environment, they need a lot of swimming space and clean well oxygenated water. Though a bit on the nervous side, they are a very interesting active fish. So a 75G would be just fine.
2.)They only eat live foods, and they prefer fish! There is a chance they can be trained to accept pre-killed fish, but you can't count on that. Because live fish are what they like to eat, a good practice is to setup a small tank to keep a steady supply of small, live feeders available( a buddy of mine uses guppies). You can also offer them earthworms, river shrimps, and other good sized invertebrates.
3.) I have no idea actually. but i have had red tail for a brief time. but my brother actually broke my tank and they all died
anyways it would have been better to have atleast 5 so they could school plus are less skittish. From my experience Red Tail hang at the mid or upper level of the tank, so a cichlid bet 7-10 inches would be good. A Geo maybe or a Severum would work. Personally, I would try Salvini or a Male Con but they may get too aggressive-hit and miss with them. So if you want to be on the safe side go with Severum. I have a buddy who has Severum with Hep. Odoe-and they are very similar in behavior to Red Tail. Why don't you just get more Red Tail that way you have a school.
other info.) Due to a diet of live foods, the Red Tail Barracuda can put a large bio load on the aquarium, so there needs to be ample filtration. Water changes of about 30 - 50% are needed every other week, depending on the bio load, to keep this fish happy and healthy.
~Joltz