I find that the best thing to do with both Gallic factions after the inital defeat of the rival Gallic faction is to disband as many units as possible, and have really small standing armies, made of cheap units like slingers and Lugoae levy spearmen. No real army at all, just garrisons and family members. But I have forts with one-unit garrisons at river crossings (preferably badly-depleted units) to stop anyone from surprising me. Especially I want to stop enemy family members getting in.

This lowers my army upkeep to absolute minimum, so I can spend money on my economy, and also save up at least 50,000 mnai. When I'm finally invaded, I aim to have enough mnai in the bank to hire a full stack of Gallic mercenaries in one go from the mercenary pool - since hopefully I've prevented any enemy FM from hiring the merc units.

So, with 50,000 mnai saved up - hey presto, I can have an instant army when I need one! And an army I haven't wasted thousands on upkeep on when it's not doing anything.