Dude, you have to be smarted than that...
Those "preset abilities" all have their ability ID. So if u made 1.000.000 abilities based on "slow". Than by issueing an order [cast slow] any of these 1.000.000 abilities can be casted because they ALL have the same ID as slow.
So:
Unit enters a region
If boolean - unit X is in region
If boolean - triggering unit is an enemy of Unit X
Than issue cast [Slow/or any other id] on triggering unit