As said above, there are many types of gouramis that get all different sizes. If you are going to put 2 dwarf gouramis in a tank, it should be a good size tank, like 20g because males can sometimes fight. Above though there are a couple people who have more than one male and don't seem to have any problems. I wasn't that lucky though.
Blue gouramis are probably the most aggressive, IMO, but most of them won't kill other fish, they'll just chase them. My Opaline/Blue gourami chases my neons.
Pearl gouramis are very pretty and probably one of the most peaceful types. Honey gouramis are supposed to be peaceful also.
Some gouramis get hugh, like Kissing gouramis and Snakeskins, so if you have one in mind to get for your tank, I'd ask here just to make sure you have the appropriate size tank for the one you get.
If you want more than one and your tank is big enough, I'd get a trio. The aggression between the males (usually all you find in LFS) will be spread out more with 3 then it would be with 2.