So, we know why Dana and Louis act so differently, but that leads me to ask, was Louis Tully ever stalked in the same way Dana was? Perhaps not having Zool fry his eggs for him, but was there a little more investigation prior to possession?
I know this information isn't in the first film, but is it ever mentioned in interviews, comments try, novelisations etc.?
In a word, yes. The Richard Mueller novelisation of the film states that the 'Terror Dog' was specifically looking for Louis and that while it was aware of his shortcomings, he was its sole target
The terror-dog had made its way down the side of the building and stepped through the bedroom window, shedding glass and broken mullions like flies off armor plate. The quarry was close, the one with whom the great transformational joining would be resolved. It was here, within this very area, but not within this chamber itself. It sniffed a few coats on the bed, then sprang to alertness as Louis Tully opened the door. It is him, the guardian thought, but before it could act, Louis tossed the coats over it and left, slamming the door behind him. It is nearsighted, decided the guardian, shaking off the coats. It is ugly, too, but I have my duty. It let loose a terrible roar.
Presumably he (like Dana) has been prepared for demonic possession by being in the vicinity of so much psychic energy. That would also explain why he's been buying headache tablets by the gross.
“I have acetylsalicylic acid but I get the generic from Walgreen’s cause I can get six hundred tablets for thirty-five percent less than the cost of three hundred of the name brand. Do you have a headache?”
According the the Wiki page the dogs were looking for a host and found Louis at a party, cornered and possessed him. So it would appear he was not stalked.
