Here's something I came up with to help decide what size to make your squads, strictly from a "sustainable casualties" point of view:
6 FW:
2 dead=25% check
4 dead=below 50%
7 FW:
2 dead=25% check
4 dead=below 50%
8 FW:
2 dead=25% check
5 dead=below 50%
9 FW:
3 dead=25% check
5 dead=below 50%
10 FW:
3 dead=25% check
6 dead=below 50%
11 FW:
3 dead=25% check
6 dead=below 50%
12 FW:
3 dead=25% check
7 dead=below 50%
12 FW +2 drones:
4 dead=25% check
8 dead=below 50%
As you can see, from this point of view (the ability of varying squad sizes to kill is a dead horse in other threads, please don't bring that up here!), 6-7 fire warriors is a bad choice because the number of casualties that forces a break-check and the number of casualties that makes a unit non-scoring are ridiculously low. The 50% check on an 8 man squad is a little better, but if you're going to buy those two extra fire warriors to gain that benefit, go all the way up to 9. For one more model, you get an improved 25% check, too. In fact, 9 is the smallest squad that gets the maximum possible 25% check, if you don't take drones. This is a good buy, from a "sustainable casualties" point of view. 10 gets you an improved 50% check, 11 is worthless, 12 another 50% check bump. To get another bump in the 25% check, you have to go to 13 models, that means drones. If you're going to take any, might as well take 2, especially since every even number (like 14) increases the 50% check. The only way to continue this would be to add an ethereal, but since he makes them fearless, only the 50% checks for scoring unit status would matter at that point. This scale could also be extended for kroot, but I don't use them, so I didn't bother.
Thoughts?