The recruitment system of the rebels is not fully understood.
However, I can say that each rebel faction (described in each region description) shares the recruitment three of the default major faction assigned. For example:
Code:
Central_Gaul
	Alesia
	gauls
	Arverni
That means that the Arverni, as gauls, will recruit all the gaul units whose owner is described as
Code:
ownership        gauls, slave
As a result, the units marked only as
Code:
ownership        slave
are not recruitable.
I hope this helps you.