Oh yeah. They are really aggressive with one another. That's why you need high numbers of them to help spread out aggression. If you get less than 12 they will kill each other until you only have 1 left.
You can keep one in a tank though. I have one in my set up. He's a punk but he doesn't hurt anyone.
Yes it is you either buy just one Demasoni or a colony of Demasoni say about 12 to spread out the aggression also on-line will be your best price solution i have seen them on Blue Chip Aquatics here is the link http://www.bluechipaquatics.com/stocklist.html for 4 bucks each on sub adults best price so far scroll all the way to the bottom..
As for tank mates you could add the Yellow labs at least 6 small individuals to start and a small colony of 6 Rusty's this will set you up for a nice looking action packed aquarium but the size of aquarium will play a role go as big as possible on your setup, i love the look of Compact strip lights with the 50/50 bulbs will pop the Demasoni colors as for filtration go with the canister big enough to do the next size tank you end up with you can also add a couple powerheads with sponges attached for water movement, marineland stealth heaters are an awesome investment as well..
Have fun my hobby friend and remember we want to see photos..