no need to change firmware in the creator x.
I never have and haven't missed anything.

Tolearance wise it depends on a number of factors.

what the original designer printed it with. if he used abs and you used pla - your's will be different dimensions to his.
What brand of filament you use, how hot you print, what the conditions in your workshop are: heats humidity etc.
There are so many variables, it's really just trial and error until you know what works on your machine in your workshop and what doesn't.

I did some nuts and bolts recently.
Without modification they were a very tight fit and I needed to sand them to get an easy to turn fit.

second set i printed I reduced the bolt by 1% and enlarged the nut by 1 %.
Came off the bed perfect, nice looses fit that still tightened and worked correctly.

So start at 1% reduction and see what happens.