We also do the temp. big sis thing--we're called Rosebuddies.
As far as taking on a lil sis, I tend to think that it's more of an issue of how comfortable you are with it. A friend of mine in another sorority took on a lil the semester after she pledged, and it worked out great. The girls we took on in COB that year (fall '02) were matched with older sisters. Dunno why, but whatever. I think exec didn't want people to get confused about how your "little" could be in your pledge class too.