Thanks, would you use missiles or beams on fighters?

I heard that they couldn't launch or fight if they were in PDCs.

OK another question concerning military vessels.  I heard that fighters wont work if they are based out of a PDC should I use carriers for planet protection instead?

No but mineral transport is pretty easy.

Mining modules only cost as much as regular mines, with some premium from other ship modules (armor and crew quarters). They're way better bang for the buck (and especially for the Corundium) than automines, but they only work on a limited number of bodies. 

Also you can assign a +mining captain to an asteroid miner, the bonus of whom will stack to the "planetary" governor (and of course sector command).

Bottom line: If you have asteroids with a decent mineral occurrence (and I always have had those), asteroid miners can be a great choice, irrespective of tech level.

Hmm.  I mean, that's a more complicated question, and it really depends.  If you want to use automines instead of asteroid harvesters then you'll need a bigger fleet of freighters to move things around.  And you can't give them automatic orders, or at least not as easily.  You could put out contracts for your civ freighters to move things, too.  But ultimately it's probably easier to just park a fleet of a bunch of asteroid miners out there and only bother to move the mass driver.  But again, that's not an efficiency answer, that's an "I'm a lazy sack" answer.

If the question is really when is the break-even point in minerals harvested if you build a miner ship instead, then you'd have to factor in freighter transport costs with automines, relative density and accessibility of minerals.  That gets into the sort of questions that actually make up the gameplay.

Unless someone has actually done the breakdown, with which case, gimme yer spreadsheets!

5 cargo holds for 1 mass driver (standard small-size installation).  Could split it up among multiple, I suppose.

Re: the tech levels, I believe that the asteroid mining module levels up as you research better mining tech.  No need to replace or refit anything.  It just increases it's production by 20% from 10 to 12 with the first tech, same as mines or automines do.