I hope with such a large group you're scaling the encounters to match them, otherwise they'll cake-walk through the entire dungeon.
Beyond the prep-work of scaling, feel free to ad-lib as necessary to make the encounters challenging to your party: have reinforcements arrive, "spot" a bad guy an extra 10-20 hp, give someone DR 2/-, that sort of thing.
In short, do what good GMs always do: "fudge" the game. After all, you bought the module, it's yours to do with as you please.
As far as trollhounds go, IIRC the bard using Obtain Familiar gets him a bat or a rat, not a trollhound (even a pup).
If he wanted to take Improved Familiar (a feat, I think?) then he can suck up the level penalty just like someone with - say - a blink dog familiar.
Also, familiars generally have a negligible presence in combat, and their improvements by caster level reflect that. His trollhound pup is going to be FAR, FAR weaker than a trollhound animal companion.
As far as trollhound pup stats go, I'd treat it like a template. Reduce ability scores, reduce HD (and therefore hp, BAB, saves, skill points, etc), reduce size (and therefore melee damage), stuff like that.
As for the druid, it sounds like the druid is going out of her way to make a trollhound work (taking a prestige class that will let her have magical beast animal companions, etc), so let her have it AFTER she qualifies for it. Even so, a trollhound probably counts as an improved animal companion (it's tougher than a wolf, for example), so it would probably have to suck up a penalty to abilities like other improved animal companions.
Also, remember that trollhounds (and animal companions) are not robots that do what the character says. They don't like going in spooky, underwater, or otherwise unsafe places - and that's what the "push" function of the Handle Animal skill is for. Since trollhounds are magical beasts, there is (IIRC) a penalty to making Handle Animal checks to get it to do stuff.
Also keep in mind that animal companions can only do stuff if they're taught tricks, and then only the stuff that the trick says they can do.
(this is why all the trained rats on level 6 list what tricks they know)
Training an animal costs time and/or money (especially if you get someone to do it for you). Don't forget trollhound feed and water, too (unless the heroes give the animal "time off" to use Survival to forage on their own - assuming it has Survival).
Finally, if your heroes are walking around with such exotic creatures as companions and familiars, that may get noticed. That means plot points.
Ankiel (or some other villainous NPC) is bound to hear about someone in town with trollhounds, and would make the connection with the heroes taking out the Iron Manacle.
That drunk dude in the Inn of the Slumbering Drake will probably have PTSD flashbacks or something at seeing a trollhound, since he was a captive who escaped. Similarly, people in the know might not trust the party.
If the party gets to the Far Garden on level 8, the fey might look down on such unnatural animal companions, or the wizards of the Tome (with whom Baghamaya on level 14 is affiliated) might get jealous that your bard has such a nifty familiar and demand to buy it from him/steal it/whatever.
When the Pack of the Night-Wolf gets sicced on the party by the Thane of Narborg, they may try poisoning their animal companions to weaken them, or maybe they'll secretly kidnap them and retrain them to turn traitor!
All just ideas to use this decision as a springboard for interesting stuff in-game.
Kinda rambling, but I hope it helps. All just my opinion, of course, but the bottom line is to do what makes the game fun (and challenging) for both you and your party.
Co-Author: The Almanac of the Endless Traders, DCC #13, DCC #29, DCC #49, DCC #51, DCC #52, DCC #63
Author: DCC #55: Isle of the Sea Drake, DCC #61: Citadel of the Corruptor, more to come....