allegiance changes

Try this
researchModifier
modifierType "AllegianceFromCultureMaxPerc"
But you need to creat a invisible and instant spread culture, I don't know if it's possible.
  

I think (not 100% positive) the global allegiance growth rates are set in gameplay.constants.

But if you just want research to increase the max allegiance, HammerSun's got it. Research to increase growth I don't think is possible.

so where exactly would i have to put the researchModifier? I haven't really done any serious modding....only changed a few things so im still rather new at this.

Open up one of the existing RESEARCHSUBJECT entity files for the general format of the research topics To create your own, you'd make a similar file (easiest is to copy and modify an existing). You'll be able to change its position (block, row, column) to occupy an empty slot, set other research prerequisites if you like, and that's where you use the researchModifier bit of code.

Then you just open up the player(tech/psi/phase).entity file, add add your new topic to the list of techs that race can research. If you did everything properly, it'll show up on the tree, and as you can see it's pretty simple

The advent already have a tech that increases max allegiance, so you can find its researchsubject entity file and look for the syntax of that researchmodifier for your own subject

Em, just as Annatar said.
I think it's not a easy work.
Could you let me know what's your essential purpose?

If you want credit grow fast, it can be changed before game load, if you want all planets allegiance high, you can change it in gameplay.constant.

If you add this research only to one or two race, will make the game un-balanced.

If you want make high income through few planet, change population in planet.entity is a better way.

Just my personal opinion,