Well, the DMG puts the benefits of being a paladin mount at +2 ECL, if you wanted to use leadership to have a beefier mount. The benefits that a familiar gets and gives are roughly equivalent, so I'd think that with the leadership feat, you could have a familiar as a cohort. I personally would probably limit this to familiars gained through the improved familiar feat.
I would think that you'd still use the master's HD for spell purposes and the like, but that you should probably change the bit about a familiar's hit points to read like this:
The familiar has one-half the master's normal hit point total (not counting temporary hit points) rounded down, or its own hit points, whichever is higher.
So, with that you could have an Ice Mephit with 1 class level, assuming an 11th level master with sufficient leadership score to aquire an ECL 9 cohort and an Ice Mephit via improved familiar. (Ice Mephit (3HD + 3 LA) + 2 (for being a familiar) + 1 (for class level) = 9).
I would be apprehensive about allowing any familiar without a listed level adjustment to gain class levels, as it could be quite tricky to adjudicate what it's appropriate LA would be.