Warning: long pastes

I want the militias and rebels to use the pirate kodiac, not the TEC kodiak.
From gameplay.constants:
rebellionInfoCount 2
rebellionInfo
minTimeBetweenAttacks 480
maxTimeBetweenAttacks 600
arrivalDelayTime 20
spawnShips
minFleetPoints 20
maxFleetPoints 25
requiredShipCount 0
randomShipCount 5
randomShip
type "FrigateTechLight"
weight 2
randomShip
type "FrigateTechLongRange"
weight 6
randomShip
type "FrigateTechHeavy"
weight 8
randomShip
type "FrigateTechAntiFighter"
weight 3
randomShip
type "FrigateTechSiege"
weight 3
rebellionInfo
minTimeBetweenAttacks 450
maxTimeBetweenAttacks 570
arrivalDelayTime 15
spawnShips
minFleetPoints 40
maxFleetPoints 50
requiredShipCount 0
randomShipCount 5
randomShip
type "FrigateTechLight"
weight 2
randomShip
type "FrigateTechLongRange"
weight 6
randomShip
type "FrigateTechHeavy"
weight 8
randomShip
type "FrigateTechAntiFighter"
weight 3
randomShip
type "FrigateTechSiege"
weight 3
This defines the two ranks of rebellion research. Simply replace
randomShip
type "FrigateTechHeavy"
with FrigatePirateHeavy, and you're set for Insurgency.
For militias, it's trickier and longer because there are quite a few militia templates. But, for example I'll pick one. From galaxyScenarioDef:
planetItemsTemplate
templateName "Template:LocalMilitiaStrong"
subTemplates 0
groups 10
...
group
condition
type "NoPlanetOwner"
param ""
owner "RandomMilitia"
colonizeChance 0
items 5
item "Tech:Frigate:Light"
item "Tech:Frigate:LongRange"
item "Tech:Frigate:AntiFighter"
item "Tech:Frigate:Heavy"
item "Tech:Frigate:Siege"
(I skipped a few groups to get to this one). Simply replace Tech::Frigate::Heavy with Pirate::Frigate::Heavy. You will have to do this in every instance of Tech::Frigate::Heavy that shows up in any militia template. A lot of them are various random spawns, and some are used in the in-game map generator that also creates militias with buildings.
Same with the TEC gauss too. In my game the gauss is lethal, so I'd rather the pirates didnt use it. I guess I can change the playerpirate file to use one of the other race guns instead.
There are better ways of doing it. First, you can leave the original gauss entity file intact, and simply create a copy with another name and mod it to whatever stats you want, and add the modified one to the TEC's build list. From playerTech.entity:
Page:1
count 6
entityDefName "PLANETMODULE_TECHORBITALGAUSSDEFENSE"
Just replace PLANETMODULE_TECHORBITALGAUSSDEFENSE with whatever you name your new file, like PLANETMODULE_TECHORBITALUBERGAUSSDEFENSE

Or you can do the reverse, give pirates/militias modified one, but as you saw above and with some militias being able to spawn gauss defenses, it's a lot easier to create one for the TEC and leave the original intact.
This should help more than my first post did

Also note, though, that the pirate heavy cruiser has no shields and no shield mitigation, so it will make insurgency/militias significantly less difficult. You can just as well use what I described for the turret, make a new entity just for the TEC, and leave the militias and insurgency with the default. I would recommend this way, but it's up to you.