I hate this happening, so I use a simple rule of thumb. I do not field armies with more than four melee units.
As for pioneers, if this were my game, I'd give pioneers a ranged weapon 'crude sling' with a damage of 1 and an initiative penalty of 10. It's not going to change game balance, and it will fix a few gameplay problems.
Parrottmath, you are the to-go-craftsman when it comes to getting solid and simple mods for things that belong in vanilla. Any chance you add this to snaking and bridge mods, and release it as the what-should-be-in-vanilla mod?
(Yes, I do know that slings are a powerful weapon that was better than bows for a long period of history, that it requires a lot more skill than almost any ranged weapon, and that this implementation would do it injustice. This is a gameplay fix)