Is Kingmaker, there were no setbacks between an npc team and a custom one (besides some missing dialogs).
Is this the case for WOTR?

Well the custom mercenaries aren't on par with the designer characters in terms of base stats.

If you look at this calculator https://pittsburghpfs.com/resources/point-buy-calculator/

A merc starts with 20 points to allocate to stats in character generation. This is a 20 point buy "Pathfinder Society" character.
A player character is a 25 point buy which is the "Epic Fantasy" setting standard.
The designer characters are around 30 or more... with one of them (Arueshalae) sporting stats that would aren't even possible in the point buy system (Higher than maximum starting stats) but if it was it'd have to be close to 60+ point buy.

I think it's possible to partially respec the designer characters to level 1. So they're mostly customisable.

If you'd rather not have the companion personalities and dialogue I'd recommend the ToyBox mod which would allow you to customise point buy mins and maxs the way you want it. And while you're at it you could tweak just about everything else, too.

https://www.nexusmods.com/pathfinderwrathoftherighteous/mods/8?tab=posts&BH=0

You unlock mercs when you reach the first safe house.

Arueshalae is a sucubus (not a core race but a 'very powerful race') and would get ability modifiers because of that. If a GM would let you play a sucubus it is possible to start with higher stats but requires a level adjustment, meaning such a character would be more powerful than your average level 1 character.