no, you most definately can not do that actually. i wouldnt recommend keeping more than 1 silver aro if you want to maintain one to full prospective aquarium size in a 240 gallon. remember, with proper diet, this one aro is gonna get 3-4 ft in that tank. you can keep a silver in a tank this size for a good length of time, perhaps the length of its life depending in what size it attains (though a larger tank for 1 silver aro is recommended=300+gallons). To answer your next question, you can keep the aro with things that it cant easily eat for a length of time. However, once it starts growing, you will need tank occupants that will grow with it. A managuense should be alright with a silver, but i would make sure the silver aro is fairly large in comparison to the managuense, considering managuenses historically display a bit more aggression than the aro. I keep my aro with a spotted gar, a royal clown knife, a datnoid, sengegal bichier and a tiger shovelnose. There is too large a list here to specify which fish could live with a silver because there are so many, message me or post with any specific questions if you like. Third question: It is fairly easy to get young silvers on pelleted food. If you do so, make it a carnivorous food and supplement with frozen and live foods but keep the pellets as the staple for convenience, if possible. The preferred diet of silver aros are live crickets, earthworms/bloodworm blocks (depending on size), krill, some will accept beefheat, pellets/sticks, and feeders (though you must be careful of infected feeders). Remember, Silvers are peaceable as long as then cant fit anything into their mouths very easily. Think some bottom dwellers that get large as well, but think sparsely. Even some large growing omnivorous or herbivories mid-dwelling species would be fine, it just depends on what direction you want to take the tank. Silvers arent overly aggressive, they are just hungry. Size is the main obstacle you are going to run into in figuring out tankmates. If i was you, id research a bit, pic up some small guys that will eventually grow large and live happily in a 240 and go with it. Good luck with stocking that monster tank!