The special rules have to share. If they didn't, you're not doing the combining fire correctly.
Let's say I have 3 units of fire warriors with a pen chip commander attached and have selected monster hunter for that turn and I chose to combine fire. First step of shooting is to select a target. When I do this, combine fire kicks in and I select the other two units of fire warriors to join the attack. I then select a weapon to fire, the pulse rifle. I then resolve the shots simultaneously. The weapon profiles have no difference, so I cannot shoot the fire warriors with monster hunter separate from the ones without. The 'conservative' way breaks here, as how do I select which dice belong to the monster hunter unit and which don't? The answer is easy. All the shots have the monster hunter rule.