i think mercs is a good idea. but then what would they look like. will you have a standard uniform and general kit, in which case they'll just be a customised force, or will they look like the unit/regiment they are ex-members of? in the latter case you may as well take the units from well-made addons like BAS ones (with the addon-makers permission of course
) and combine these with some of your own that you think would make a decent contribution to the unit.
Basically most of your mercenary addon would probably centre more around weapon and kit selection (using weapons already widely available as addons) and perhaps using a "buy" script (there are some of these around) to represent the mercenary army's need to fund themselves rather than have government and tax-payer backing.
I think the best idea for a mercenary army, therefore, is for someone to go around and collect decent addons from around the OFP community. Then, with permission, edit these to get a general theme across all the units so they fit together (renaming them "ex-ranger, woodland gear" or some such name, removing regimental insignia). They should then release this as a merc pack together with a good weapon selection, purchase scripts, etc.
This way, so long as you get permission (I know i keep saying this!) you end up with a good collection of the best addons out there taken from what may be larger packs giving a decent variey of weaponary, etc.
in case i haven't mentioned it, you'll have to get permission of course ....