The only manly fish I can think of that would fit into a 10g tank (a 20g long would be much better) would be the exodon paradoxus. It is an extremely vicious tetra that will kill just about anything put with it. Only about 6" long, they are fast, toothy and mean (a distant relative of the pirahna - both are tetras). If your pet store does not have them, they can order one for you. If you can't have a tank of at least 40g, only one fish is advised. A school of at least 10 is recommended in a larger tank. They kill each other off.
Here is some more info on it:
http://www.aquaticcommunity.com/fish/bucktoothedtetra.php
In the wild, they travel in schools and don't have the aggression that they have in a closed environment like an aquarium.
Live food on the order of worms (bloodworms, blackworms), brine shrimp or guppy fry would be about the size that it could eat easily when young. They will accept flake food too, but you must start them on it and then offer the live foods later on. As they get larger, you could feed them feeder guppys.
I worked at a pet store for a while and just one of these cleaned out a whole section of our live fish display tanks. Nothing could stay alive with him. We sold him three or four times and each time he was returned. I went back a year after I left their employ, and he was still there....alone, in his section of the display.
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