I don't know why they shouldn't fit. NiMHs are mostly equivalent to NiCds besides the higher capacity. You would have to load them longer (twice the capacity=twice the load time).
The HP charging circuits load with some kind of more or less constant current (in fact they load via a resistor coming from a constant voltage source (7806 IC)). This is the same method used in the cheapest external charging devices. I mean the chargers you get for some $ everywhere.
I don't see a difference. When it is cold, they lose capacity, which maybe can be a problem in winter time.
The benefit is, that they don't have any memory effect.
They should also not be left on the charger for infinite time like much users treat the NiCds. This is also no good practice for the NiCds, but probably the NiMHs react bad. (get hot, I think)
I'm not a battery expert, but when used the right way, I don't think they can do any harm to the calculators.
I think, I have to check this.
Anybody else out there (some electronics engineers, I know you're here, guys) who knows more ?