Well it's not so much a question of what will WORK, it's what will FIT and what connectors will be compatible. You could hook it up to a car battery for for gods sake :L
So, any battery (NimH, LiPo, NizN) will work, if they have the right connectors. I believe that gun has Tamiya connectors, so you need a battery with Tamiya connectors for them to be compatible (So yeah, I guess you were right, work!). Not many LiPo's I've seen have Tamiya's so you either have to change the LiPo's connector to a Tamiya (Not recommended) or what I recommend, change them both (Gun+Batt) to Deans (Loads of YT Tuts), as they are safer than Tamiya's, and give better trigger response, ROF etc. Now, size. If you have a standard M4 handguard and wired to the front (Which you have), you'll need a slightly smaller battery. If you wanted NimH for example, you'd need a Mini Battery. For LiPo, I don't think it's necessarily like that. I think, depending on if it's abnormaly long or not, a Nunchuck LiPo, should fit? Don't quote me on that though :/ Sorry, I can't help much on the size, you'll have to leave that to someone else.
For brands, Turnigy are great from what I've heard.
Have a look on HobbyKing for all your LiPo needs :)
Here's one of their batteries. I think that should fit, not 100% though