As you stated, we're going to have the chance to get Lin's sister, and that's cool. You've also just suggested buying the bitchy high elf, and I gotta say, I LOVE that idea. Someone to be bitchy and angry at us while we get to play with them sounds like a lot of fun. These sound interesting because they both have a story to them.
If you want to give us any other elves, I'd want them to at least have a side-character level of story to them, like those other two; something to connect them to the player or one of the other girls, either before or after being purchased. If any of them were just like the elf slave of the other owner we met in the clothing store I don't think they would be very enjoyable/interesting.
With that in mind I also wouldn't want a harem of elves competing with the main cast either. Maybe no more than 1 or 2 others in addition to the two that we already know. I wouldn't force it either. If you happen to conceive of some new side-character that could work as an elf, then it could be fun. Otherwise, don't worry about it I say.