It sounds like you already cracked open the RESEARCHSUBJECT_MAXSHIPSLOTS{Whatever].entity files, and that you've messed with these lines (As taken from TECH0):

researchModifier
modifierType "ShipMaxSlots"
baseValue 100.000000
perLevelValue 150.000000

If you add something like that to a different research file, it should add that effect. Just remember to up the count on the researchFloatModifiers since you're dealing with Floating Point numbers.

Just for your new research changes, leave the baseValue at 0

Too bad... Let's hope it will be in the complete mod tools when they release it.
I was thinking of the star trek armada system as well.

@ Bink

i think you misunderstood what i'm trying to change.

I want the planet civilian infrastructure upgrades (as they are called ingame) to add fleet cap. Its one of the planet upgrades, like the tactical/logistic slot upgrades.

from the planet entity files :

planetUpgradeDef
path:Population
stageCount 4
stage
price
credits 0.000000
metal 0.000000
crystal 0.000000
upgradeTime 0.000000
maxPopulation 10.000000
populationGrowthRate 0.125000
developmentTaxPenalty -2.000000
stage
etc..

OK i found a way to add to max fleet capacity without research - wonder why it took me that long ... (in Research known as ShipMaxSlots )

just make a ship entity and use a negative value e.g. slotCount -25.000000
Much simpler: make the Tradeship have a negative slotcount like -10 .

Or mod a slow thing like an expensive command cruiser or an orbital depot (speed very very slow just to move out of your capital ship yard) without weapons (juicy targets that give the attacker lots of XP) and for a onetime fee you have additional slot counts . Now you can mod your orbital Starbase.

At least an enemy fleet cap can be better reduced than if its based on planet.

Hope it helps !