You're not going to fly SEL for the airline so no need for the rating. You don't even need to get the commercial MEL. As pointed out above, to fly for an airline in the US you need an ATP. To get an ATP you have to hold a commercial certificate with an instrument rating. You say you've got that in rotorcraft so you're good to go. The reg does not require the commercial to be the category or class as the ATP.

I don't understand your comment about doing the "entire program or just the classes." In FAA land, EVERYONE (unless you hold a Canadian ATP) has the same route to an ATP. Complete the Certificate Training Program, pass the written and pass the checkride. If there is a type rating involved there is an additional step. For a type there has to be a record of ground and flight training before the checkride and an instructor's signoff.

1500 hours won't get you in the door at a major unless you're military.