Can I tap fewer than 7 creatures with Lullmage Mentor?
No you can't.  due to rule 117.3


  117.3. A player can’t pay a cost unless he or she has the necessary
  resources to pay it fully. For example, a player with only 1 life can’t pay a cost of 2 life, and a permanent that’s already tapped can’t be tapped to pay a cost. See rule 202, “Mana Cost and Color,” and rule 602, “Activating Activated Abilities”




the text: "tap 7 merfolk" is before the :, and therefore it's a cost.  You can't pay or attempt to pay that cost unless you are able to pay it fully.