(sorry old post)
Personaly, i slapped on an external tank used o regular shuttles, slapped 4 engines on the botom of that, and added 4 shuttle SRB's (witch was overkill if i remember correctly), and mmade sure that the pathfinder tanks did not interact with the ET, so that i had a full tank of gas in pathfinder.
i know its kind of a "bad" ( i guesse?) alternative for air launching, but to be fair, i played with this mod when the suttle's aerodynamics were kinda screwed (if they were always fine, i guesse i'm just a bad pilot :p)