Standard Askileon Military suggest 1 healer per 10 for Cavalry and Infantry (and melee special forces) or 1 healer per 15 men for archers and ranged special forces. We generally frown about Mixed Infantry and I have yet to see anyone pick such a unit, until someone does we dont have a suggestion 
If MI are on a testing island, treat them like archers; if they are on stable, treat them like infantry. Or just give up because they will break rank and charge the enemy and be cut down.
