This is incorrect. You only need 3. Make sure the satchel lands on the truck itself.
Penals are great units. The only thing wrong with them is the fact that they fulfill The same role as conscripts. Penal starts give you a weak early game, since you are behind mp wise, but gives you a boost mid game. You just need to keep them from n groups of two, and flank enemies. 2 squads will melt any opposition until obers arrive.
That is weird, I took six, and did three experiments. I will try again.