I'm currently painting a women's battalion for my Zapatista forces using the OG figures.
Handily, the pack size just happens to align perfectly with the unit sizes in my favoured ruleset, CLA, but this goes for the range generally: my Federal infantry units are fifteen figures strong because it makes sense historically, and because you get thirty figures in an OG bag, including two officers, two NCOs, and two buglers. Cavalry are ten figures strong because that's what OG gives you, and historically units tended to be around two thirds the strength of infantry units at both squadron and regiment level. The only fly in the ointment is the dismounted cavalry packs, which despite containing thirty figures, include only two officers and buglers.
With this scaling system one figure represents about 25 combatants.
Rurales units tended to be smaller, so are the CLA minimum unit size of eight figures strong, which doesn't align with the OG pack size of six figures, mounted and on foot, but works out OK for the dismounted component because you don't get command figures in the OG pack and have to add them from another source. I used Outpost officers and buglers.
For the mounted component there's really no way to avoid buying an extra pack to make up the numbers, but there's no need to worry about left-over figures because any extras can be absorbed into your revolutionary forces with nothing more than a different paint-scheme, or if you don't mind the extra effort, a head and/or weapon swap.
Volunteer units were also usually small, so once again they're eight figures strong.
You should probably have no more than one volunteer/militia/auxiliary unit on the Federal side in a game, as they were normally very much in the minority.
For Rurales multiple units are realistic; sometimes they were the only Federal cavalry present in an engagement.
There were also a couple of Rurales infantry battalions in the Federal establishment, probably constituted from men who'd lost their horses. You can make these from the OG figures by carving their sword scabbards into bayonet sheaths. Conveniently the OG Rurales figures are sculpted with rifles (as also are the regular Federal cavalry). For my mounted Rural and regular Federal units I converted them into the Mauser 95 carbine with which such units were historically armed. This isn't too difficult thanks largely to the fact that that weapon was fully stocked to the muzzle; just snip off the end of the muzzle and file it back as far as possible whilst leaving a thin strip of the forward barrel-band. File this down somewhat, but remember to leave enough at the top to create a foresight.
Back to the ladies: I did one head swap between two of them for a little extra variety.
If you want to do an entirely female unit yourself it's really only realistic for the Zapatistas. Although many women fought as individuals with other factions, I'm not aware of any non-Zapatista units as such. And as with volunteers etc. there should be no more than one soldadera unit in your Federal army.
