I would also note that Stellar Conquest + Starmada would be ideal with a referee, though I've been thinking about 'detection' and 'dummy' fleets for straight 2-4 player games.  IF anyone would like to test the concept, I might be up for being the referee of a test session, say within the next month or two.

There are few main 'sticking points' to mesh the two, beyond determining how production points should map to Starmada's point systems:

1. How to handle planets.  Starmada doesn't have a ground assault element, but perhaps simply creating a 'ground assault' module to install on Starmada ships would work.  Then use the Stellar Conquest ground combat system (I don't recall what it is off the top of my head ) if any of the ground assault ships survive the battle.

2. Refits... This adds an additional wrench into the ship development, prototype concepts mentioned previously.  Adding or replacing some systems should be relatively straightforward.  For weapons I tend to think that any weapon type could be replaced by any other type as long as its space requirement is within a certain percentage and the arcs are the same.  Again, a prototype of the refit would have to be made first.  The normal production cost to refit an already constructed ship would probably be something like cost of the unit being installed * 2.0. 

As a note, I have a vague idea that something like the B10's development history in Star Fleet Battles could be easily simulated here... A huge ship might not be finished in one production turn, so it could be refitted while it's being built, upping the cost further until finally someone says 'enough already, finish the thing and get it into battle... This might be too complicated to deal with at first though, as the specific items on the ship would have to be indicated rather than just... 'Ship costs 200 production points to fully build, I can only put in 50 a turn...' which is much simpler. )

3. How to handle various types of non-weapon tech improvements/systems.

4. Min/Maxing... In other words, given a certain overall tech level and a starting pool of production points to research and build an initial fleet (tradeoffs here between more ships or higher starting technology), is there an obvious set of weapon traits, ship types that everyone will gravitate towards.  An alternative would be for each player to pick a trait, system type, etc as their 'base technology' which would then be unavailable to anyone else at the start of the game (but could be researched later).

I remember ADB did a Campaign Designer's Handbook for SFB a few years back.  It included different tech trees (phasers, bigger ships, photons, drones etc) with each race having stuff on the various weapon trees that they had already developed (so the Feds got some proximity photons, the Kzinit got the first drone speed increase etc).
You could adapt this, depending on how complex you want to get, so you research gradually longer ranged weapons, higher impact, more accurate etc with weapon traits being offshoots of these.  You could then have another tree for fighters/strikers/seekers (maybe you have to "invent" seekers before you can develop strikers), another for engine size, and another for ship size.
That would allow one race to put their research into small fast ships with short range but high impact/damage weapons, while another goes for big slow ships with long ranged but low-power guns and carrier capacity.

Stellar Conquest had no ground combat system. Personally I like the map, but would use the VBAM advanced system generation and economy. That makes it simple to use the conversion and even provides a ground combat system. The only thing that would need to be created would be the movement system. For that I would more or less stick with the SC rules.
I may have to give this a try!

For ground combat, why not make a 'flotilla' with no engines and no hyperdrive and go with that using the technology that the player has already learned.  I'd look at VBAM/Starmada crossover for ideas!

-Bren