This army would need a stat line for "Rabble" or former slaves armed with anything they can get their hands on. In other words, "gladius fodder", for the lack of another term---or maybe grist for the mill? I would think that they would be at least 50% of the army.
Well we know that 2 legions (10,000 men) could defeat a group of 30,000 slaves (lets assume 10,000 fighting men there) so even after training slaves still not as good as Romans. But Spartacus army of 70,000 (23,000 fighting men?) then easily beat those 2 legions.
Then for the final showdown you have 120,000 slaves (40,000 fighters?) vs 8 Roman legions (40,000). I suggest that you can ignore the women and children, they do not seem to have made any difference to the combat.
For equipment, the slaves had the weapons they looted from Capua (gladatorial training centre) and from the 15,000 (at least) Roman legionaries they beat. Plus of course anything they could scavenge/make.
I was peeking at another ancients rule book whose name escapes me and found a description of "rabble", but I can't recall the relative fighting value compared to regular troops. Perhaps the stat line for a standard-sized unit of slave rabble would be 4-4-2-0-0-6. Or, should they have reduced stamina? Some useful rules that could apply --- Levy, Militia, Freshly Raised, Wavering. They would be based as a 4-rank Warband, I think. Or, perhaps because of the desperation of their situation, they might fight harder -- perhaps Wild Fighters would be applicable? I like the variability of Freshly Raised. Thoughts?