Just have the pun-believers stay at 1 hd, and don't bother with spell likes that rely on caster level
They can still give YOU manipulate form, and then you ascend to full pun-pun power, now it is safe to raise the believers HD a little.
you are now an entire pantheon
you could even give the pun-believers the thrallherd abilites and have multiple tiers of pun-believers
Of course the real pun-pun could do this at level 1, by gaining the thrallherd abilities and believers....
still, I can't help but think of this when playing a thrallherd.