Think of it as rulesets:
If everybody picks an x-ship, then it uses this:
"All of my generated AI ships will be from the Advanced Era".
If noone picked an xship, then it uses:
"All of my generated AI ships will be ships that are in EXISTANCE during the chosen era.". The YearFirstAvailable and YearLastAvailable plays an important part.
This includes heavy battlecruisers, dreadnoughts, etc. ... all those ships continued to exist in advanced era, even with the appearance of x-ships.
If it's a mix of players (Say 4 out of 5) who picked x-ships, then:
There's a 4/5th chance that it will be 100% xships generated. There's a 1/5th chance that it will be a mixed fleet of ships that are in EXISTANCE during advanced era, including x-ships.
-- Luc