Is it possible to have skills suggested in this way?

In some cases it's not worth using certain skills unless there is more than one enemy, but would always be worth using during a boss fight.

Yes.

boss is true/false if it is a boss fight.
active_enemies is the number of enemies.
| is the "or" operator.

boss | active_enemies > 1 would be an example of that.

If there are already other criteria, you can do:

( boss | active_enemies > 1 ) & other_criteria_here