Not quite right, RCIX, but not a bad guess. Actually, I think no one has ever asked this before, which is interesting. I will have to put it in the wiki. Here's how it works:
When it comes to fleet ships:
- All players always have fighters, bombers, and frigates.
- AI players then get one unlock at random for bonus fleet ships. This is determined based on the map seed, not based on the AI type chosen, what the players have unlocked, or similar.
- AI players then may or may not get more unlocked bonus fleet ships based on their types. Stealth AI types get a lot of ships with cloaking, and so forth. If the initial unlocked bonus fleet ship would have been given to the AI as part of this stage of unlocks based on their AI type, then that's just too bad for the AI, they will have a more homogeneous fleet.
- Next the AI players each get three unlocked Core/Mark V ships. Core ships are treated entirely separately by the AI from the Mark I-IV ships, because there is not a core variant of over 2/3 of the fleet ship classes. Each AI player gets only these three initial core ship types in a given campaign, and never gets any more.
- Lastly, if an AI player is set to difficulty 7 or higher, it gets a second bonus ship type to start out the game. This would be one that has not already been unlocked, and which again is based on map seed -- but since it is required not to be one that was already unlocked, if it fell amongst those that have been unlocked based on the AI type, then it would instead shift to something else. So the AI type chosen will sometimes affect which second ship type is unlocked here, but mostly it is just based on the map seed.
Once all that is taken care of, the game starts and that's what the AI has. As far as any other non-fleet starships go (meaning Starships, special ships of various sorts, capturables, etc), that is determined as part of the initial map population algorithm based on the AI type and the map seed.
As the game progresses, the AIs will occasionally get access to a new bonus fleet ship type. This happens when your total underlying AI Progress (before reductions) passes the following thresholds:
- 200
- 400
- 600
- 800
- 1000
- 1200
- 1400
Those newly-unlocked ships that appear later in the game will be ship types that neither AI player already has unlocked, and both AI players will get the same unlock at that time. Rather like what happens with a team of multiple human players when an Advanced Research Station is captured. Which ships are unlocked here are based mainly off the map seed (with the added requirement that it not have already been unlocked by the AI team, of course), which again is the same as how the Advanced Research Stations work for the human team.