Is it possible to play with 6-7 heroes? I just purchased the original game with out expansions and am curious as to the expansions allow for extra heroes or if anyone has created some sort of house rule to modify the overlord slightly to compensate for the extra heroes.

The player scaling really doesn't work even for the range the game theoretically supports; larger parties of heroes have a significantly easier time, and the game is almost unplayable for only 2 heroes. If anything, expansions further constrain the number of heroes you can use; expansion quests tend to be harder than base game quests (making playing with small parties less realistic), and the advanced campaign explicitly only works with 4 heroes exactly.

The most common way of coping with small player groups is just to divide the desired number of heroes among however many players you have. Various quick fixes have been tried for smaller or larger parties, but none have gained widespread acceptance, and I don't really have faith in any of them.

I have a homebrew Descent mod, The Enduring Evil , which is designed to address player scaling issues (and some other perceived problems); I think it scales reasonably well for anywhere from 2 to 5 heroes, and you could probably extend the range more easily than you could for standard Descent. But it's a fairly radical overhaul; it requires printing a bunch of new cards, and isn't compatible with standard quests.

By the time you're considering seven heroes, I'd suggest you should split up into two separate games. Even if you somehow magically got the game to properly scale that far - which I'm quite confident you won't - you'd dramatically increase the playing time of an already-lengthy game.