the actual rule:
Some rare units always count as stationary when firing rapid fire weapons and some units can move and fire heavy weapons. Such units can charge after firing
This can be broken down into the following:
Some models count as stationary for firing rapid fire weapons. Such models can assault after firing.
Some models can move and fire heavy weapons. Such models can assault after firing.
The part that allows Terminators to assault after firing RF weapons ISN'T that they count as stationary for RF weapons, because they don't.
It's simply that the rule doesn't qualify WHAT the model is firing before they assault.
'Such models can assault after firing'
'Firing', without any sort of qualifier, simply means 'discharging a weapon'... not discharging any particular weapon (that would require some sort of qualifier on the end)... just a weapon.
So, Terminators, as a model with the ability to move and fire heavy weapons, can assault after discharging a weapon. The rule doesn't require that weapon to be a heavy weapon. It simply allows them to assault after they fire a weapon.
I agree with you on everything except that the rule allows to fire anything as it neither restricts nor allows a particular weapon to be fired before charging. So the rule wording:
'Such models can assault after firing'
simply gives them
th ability to charge after firing. As it does not say "after firing these weapons it counts as stationary for, it simply gives the ability to charge after firing. Since it does not remove the restrictions already in place for rapid fire weapons (it does not say 'after firing rapid fire weapons') then the rule in facts can be read to mean nothing more than giving the ability to charge, which all infantry have without counting stationary with a pistol. Going with 'wnything' is adding meaning, where as adding 'with the weapon it counted stationary for' is adding restrictive and in context words.