Page 151 of the Ars Magica Fifth Edition rulebook gives a level 20 base level guideline for Rego Mentem to give a person a complex command. Can this extend to Fae Creatures with Faerie Might?
I don't see anything that would prevent this.
Ars Magica Core Rulebook Page 148 says:
Intelligent minds are affected by Mentem spells, while the minds of beasts are affected only by Animal spells.
The only thing to remember is, that faerie might gives magic resistance.
Ars Magica Core Rulebook Page 191 says:
Might Score is very important, as it provides Magic Resistance [...]
CREATURE MAGIC RESISTANCE: Might Score
So it should be possible, but not necessarily easy.