This is not possible to do as you have it stated here.

The only way to construct frigates and cruisers is through the frigate factory and a starbase with the factory upgrade (tech).

The only way to construct capital ships is through the capital ship factory.

This is hard coded.

There other two ways to spawn/create ships is through abilities.

The CreateFrigate buff instant action type is meant to create temporary frigates (temporary do not use fleet supply). It can be used to make frigates that take up fleet supply too, however this method does not prevent a frigate from being created when there is no available fleet supply which can easily result in negative fleet supply.

The SpawnShipsAtPlanet (see vasari phase gate) buff instant action type can be used to spawn frigates and will observe fleet supply limits. However, this mechanism has all ships that are spawned phase jump into the planet. It is possible to spawn capital ships this way, but capital ship supply is ignored which can easily result in a negative capital ship supply.

Hope this helps.

Yeah, no way of adding more to the factories. You can, however, have stuff spawned via an ability like Returning Armada.

I think it has to with the addition of Corvettes, but I doubt it will be any more flexible for us. It will just add another 9 slots to work with.

Yup. You can also have a delay on the buff so that the ship doesn't actually spawn until the cooldown expires.

I thought the ability limit for everything was 5?

Indeed it is.

maybe I was too technical, but this is what I already described. You can only use SpawnShipsAtPlanet with this approach and only to spawn in frigates. Most likely the AI won't be able to make good use of this as role types would be completely lost in the section process and would most likely just randomly spend it's money on a poor fleet selection.

So you can't make frigate page 3 then?

We are stuck with a frigate factory that builds frigates and cruisers, but a capitalship factory that only builds capitalships and you can't get it to build "Battlecruisers"... no support for page:1 in player entity file?

    capitalShipInfo
        Page:0
            count 4
            entityDefName "CAPITALSHIP_TCACentauro"
            entityDefName "CAPITALSHIP_TCADRESDEN"
            entityDefName "CAPITALSHIP_TCABETAKOL"
            entityDefName "CAPITALSHIP_ATHENA"
        Page:1
            count 6
            entityDefName "CAPITALSHIP_TCACARRIER"
            entityDefName "CAPITALSHIP_TCACOLONY"
            entityDefName "CAPITALSHIP_TCASIEGE"
            entityDefName "CRUISER_TCASUPPORT"
            entityDefName "CruiserTCAUltraHeavy"
            entityDefName "CruiserTCAUltraSiege"

Oh to dream... while I'm at it I want a campaign and campaign editor

No, the frigates available are in the Player[Race].entity file, not the PLANETMODULE_[Race]ORBITALFRIGATEFACTORY.entity file.

If you are willing to use the fleet beacon spawn for the player, I may have an approach that's useable by the AI as well if you REALLY, REALLY need more than 18 frigates/cruisers...

I've accidentally done this before, but it could be utilized for something like this.