Canon doesn't really play much part in this game. I.e. Mandalmotors has a YT-2000 DC, while Corellian Engineering Corporation doesn't have any YT DCs. I don't believe the admins have any plans on distributing any more DCs, although with R&D, factions should be able to research pre-existing DCs.
Shobba, yes, the DCs that currently are assigned to factions were all doled out at/near the start of production, with very few if any added sicne then. Don't expect any more pre-R&D. I wasn't quite a faction elader when Prod came out, ut I do recall a bit of a horse-trading and balancing aspect to the DC distribution to try and keep close to canon as best possible, but also not to make any of the existing factions at the time over powered or useless.
As for R&D and future DCs. when R&D comes out, every faction will start at the bottom and can "learn" any of the DCs on their tree, per what's been said so far by the Devs. So if you start a ship faction, you'll be able to R&D every ship DC on that ship tree... btu it'll be the same DCS every other ship faction can research. If you folow different paths on the tree, its quite possible you can R&D ships that no-one else has found yet. Without knowing what the tree will look like, its hard to say how likely or possible that would be... depends on how many ship factions rush up the tree to go find the SSDs and large caps, versus others who might be looking for the better fighters or freighters.
DCs were originally handed out based on what factions were originally around when production was implemented and the last Dc's were distributed. Some have been given since the combine went into Beta mode officially near the end of Red Scenarios I believe, though I joined a couple of years after that point just prior to production being released, so I saw the last ones handed out for items etc.
*Achievement Unlocked - First ever person to beat Mandalorian Intelligence - Reward: Political victory over Despot DeMeer*
Yeah, that's another good point, faction mergers cause DC mergers. Teh current Sorosuub was not the one that merged into the NR, so old Sorosuub DCs are now in Incom... faction mergers ar the same reson that Merr-Sonn has so darned many DCs these days.