Well there arent any maxShipSlots in the gameplay.constant, however you have to alter the the ai to build x times more the ships that can now be build (found at the buttom).

This is from a mod where I increased the logistics by 10 times:

table-Aggressive
   BuildShip 100 - increased from 10
   BuildModuleTactical 2
   BuildModuleCivilian 1
   BuildModuleResearch 1
   UpgradeResearch 2
   UpgradePlanetArtifactLevel 1
   UpgradePlanetPopulation 1
   UpgradePlanetInfastructure 1
   MaxBountyBidCount 5
   UpgradeStarBase 1

no that changes how the AI will build its ships and at what rate

the RESEARCHSUBJECT entity files is where you make the changes and yes you need to do it for each race