It is impossible to tell every merc who is able to earn you support xp because there are too many. Support xp are awarded by reviving, healing, resupplying ammo to teammates and spotting. Also, Guardian earn support xp for each projectile/airstrike denied by the skyshield, and Turtle got some too when enemies are shooting at the shield.
There’s no specific merc mission anymore, those are generalists now (Support, Combat, Objective or Overall) because so many mercs can earn them (as I stated sooner for support xp).