Of course, neither are cats (who can be familiars for arcane casters but not animal companions for druids or rangers)
House rules. 
I think all of those are options, you just have to agree with the DM on the details.
Which does open up another possibility by which Mr. Scruffy might be saved: Ordinarily, druids (or rangers) can choose from a few default animals, like a wolf or horse, or they can choose a more powerful animal, at the expense of a penalty to their effective level for the benefits the animal gains from being a companion (like, you can choose between a really souped-up wolf, or an out-of-the-box T. rex). Since a housecat is a less powerful animal than the default choices, Belkar might be treated as if he has a bonus to his level for Scruffy’s companion buffs, and so he might have more HD than he’s “supposed” to.
What’s nasty is that most fiends who can use Blasphemy, get it at will. Which means that if you’ve got one fiend who can use it, and an entire party that’s subject to it at all, that one fiend can keep on trading actions with the entire party forever. And if that one fiend has any allies at all, those allies can just keep on doing whatever they do forever without the party ever getting any chance to do anything to stop them. An enemy with at-will Blasphemy is a very definite “you must be at least this tall to ride” encounter.
Though in this case, with Belkar and Serini unaffected, that might not work out so well for the Linear Guild. It’d effectively boil down to those two versus Thog and Sabine, while Nale keeps everyone else occupied, and both of the halflings fight dirty.
Yet again, though, Nale really doesn’t actually want to destroy the Order. This is just about stopping them from killing him, and getting a little revenge in the process.
1339 - Weak Interaction
Scruffy’s still alive, so that’s good. Any way Rich could have done that without serious fudging?
You may have spoken too soon.