[Inquisitor Solexus]

My guess is you were playing it that anything that floats is hit on a 6. "Skimmer" is a vehicle attribute, and only vehicles can ever claim to be one. All other models are hit on basis of weapon skill unless otherwise stated. This means things like jetbikes are hit based on weapon skill, unless they have a special rule that says otherwise.

Absolutely. Since you have to assume the jetbikers get down to ground level to reach their enemies, they're within normal weapon reach.