question regarding NPC groups

I was browsing around the forum and couldn’t find an answer to this to I figure I’d just ask: How does one stat up a group of enemy mice or weasels? What I’ve been doing is just taking a typical mouse/weasel of appropriate occupation out of the book as a base stat block and then adding +1 to Nature (for purposes of increased disposition) per individual (I try not to go more than 3 on a team, so if a base Nature is like 4, then with three of them I increase it to 6), and I generally assume that they’re all identical in terms of skills and traits, so I add +1D to each trained skill to represent them aiding one another–for example if an individual weasel has Fighter 4, he’s rolling at 5D (regardless of the actual amount of individuals) to represent aid from his comrades.

Is this right? It seems to work out, but I don’t want to be learning it (and therefore teaching it) incorrectly.