Absolutely do NOT cohabit him with gerbils. Hamsters are iffy enough with their own species, let alone others, and dwarf hamsters are aggressive enough that, despite the size difference, he could do some serious damage even on three legs.
Keeping hamsters together is very, very difficult. If he'd been together with a same-sex sibling from birth, I'd say if you have at least a 20 gallon then proceed with caution, but if they're separated for even just a day, they "forget" and will attack any other same-sex hamster in their territory. Opposite-sex is another story, but then you'd have a whooooole new set of problems: 6-12 extra problems, to be precise.
Hamsters aren't super-social like gerbils, and I assure you it would not bother him one second to be alone for the rest of his life.
I've kept dwarf siblings together from birth (back in my younger, stupider, "oops litter" days. To be fair I kept all the pups...ask me some time about raising 20 hamsters at once for two straight years! NEVER AGAIN). Some did fine. Some went Hannibal Lecter on their siblings. It's not worth the risk.
Dwarfs are smaller than Syrian (aka "Golden") hamsters, but what they lack in size they make up for in activity. Ten gallons is the absolute bare minimum size I could comfortably recommend for a single dwarf. Twenty is better if you have a spare or can find a leaker, although if you have a spare ten you can add to it with wire cage toppers. You can also make homemade hamster cages out of large plastic storage bins. I've never gone that route (by the time I learned about it, I already had enough spare cages to start my own rescue), but it's great for dwarfs...they don't have the jaw strength to chew through plastic like Syrians do. You can also go the plastic console route--you know, Habitrail, Crittertrail, S.A.M. tube systems--which hamsters love for the tunneling opportunities, but they can get pricey and lack floor space. You would need to hook two or three together to provide sufficient room.
Hamsters don't climb and jump like gerbils. They're more ground-oriented. Vertical space and levels that a gerbils would enjoy would be lost on a hamster, and may even lead to injury from a fall. They can't balance very well (if at all), and aren't remotely athletic (unless digging is an Olympic sport). When buying a cage, think floor space. Also, as burrowing nocturnal animals, hamsters have very poor eyesight. They can only see up to six inches in front of them; after that things get a little fuzzy. They appreciate lots of hiding spaces and houses.
