The jump in unit size was decided to show the change from manipular organization to one based on cohorts (which are bigger than maniples). The number of men in the Levantine Saggitarius Auxilia unit should be 80, I'll fix that in the next build. Also you're paying per man in each unit, so if you want smaller or bigger units you should adjust the costs to reflect your changes.

O'ETAIPOS is right about the mercenary_unit attribute, it's entirely a model sharing issue. The late principes model is shared with another roman unit (late hastati I think) and if you take off that attribute one unit will have the wrong skin. RTW only allows one skin per unit per faction and using the mercenary_unit attribute is just a way to have another skin on the same model. It shouldn't affect gameplay at all and it doesn't make the principes hireable anywhere.