I am vaguely thinking of doing a Servile Wars campaign. The DBx rulebooks have some slaves in captured Roman equipment, some fighting in Germanic or Gallic styles but the rest are considered a 'horde'; which figures would be best to represent these masses?

A mix of AGR3, 4, and 10 (Psiloi with Javelin, Bow, and Sling repsectively) with some AGR09 (peltasts) or even AIT01 (samnite infantry) would work well as an ancient horde.  AMO01, Moorish infantry would also work.

Individuals or small groups of AMA09 (theurophorus infantry) and/or AMA11 (imitation legionaries) on the same base would add some variety.  

thank you

Given the large numbers of Roman soldiery that were killed or captured, all fully equipped, I would expect the slaves to look very much like Legionaries, albeit without their drill or discipline.  I'd go for most equipped with gladius rather than pilum and cut the strips up to present a less rigid formation.