I guess I can't speak for anyone else, but in Alpha Phi if you are part of a collegiate chapter, you aren't in the alum chapter. So no, the alum chapter isn't made up mostly of collegiates. Maybe I read this wrong though?
Also, from what I've seen, while there is some support for the collegiates, we don't really do too much with them. I know things are different in other parts of the country, but I've never seen an AC really get involved deeply with a collegiate chapter besides planting flowers in front of the house, a little rush help, scholarships, etc. The advisers are the alums who are really involved. If I find an alum who is interested in collegiate stuff, I try to get her to be an adviser. Of course, with four collegiate chapters within 10 miles of me, we can always use more advisers!