[quote=@Hawthorne]
<Snipped quote by Caasicam>

Gotcha. Appreciate it! 
[/quote]

Caasicam mostly has it. [i]Most[/i] Orbitals are flight capable, but not necessarily capable of effective flight for combat. Not all, but most, Orbitals are able to fly a bit like a cargo transport, able to get from Point A to Point B. Orbitals meant to fly for the purpose of combat still require a lot of purpose building because they have to be able to handle so much better in the air, and not just "capable of taking off and landing".

Short version, what you're thinking of should be fine. With enough work you could just fit an Oberth Reactor into an older unit, if you really wanted to, so there should be no reason that wouldn't work. Heavies, mind you, aren't necessarily literally heavier; they're just seen as less mobile owing to older technology.