How about if I have one same ability on different gears? Will it still stack? Or for the abilities to stack they have to be on the same gear?
Yes. Doesn't matter what gear its on. It's just more convenient if its on the same gear.
Cause then I have say, my Ink Up T-shirt and can wear that when I want Ink up, and then wear a different shoes and hat for different things. Instead of having to wear 3 different pieces to get the effect I want.
Here is an example from Splatoon 1.
I used sloshers a lot, could kill in two hits. I thought about using a damage up bonus to kill quicker, and while I could up my damage and technically kill faster (say they were already damaged) no amount of damage up power ups allowed me to kill faster than 2 hits. (Meaning I approach someone at 100% health, its always going to take 2 hits, no matter how high my damage is)
I'm sure people are out their compiling the data, but the point is you can get 9 power ups, but it might night make a worth while difference in that area and you are better off putting them to other things.
The abilities stack, but it only goes so far. So you want to think of it all in terms of your weapon and what you are trying to do. You can compensate for a weapon that uses too much ink with ink up power ups etc, increase your special charge, or effectiveness etc.
Another thing people used to do is with chargers, they'd put a hell of a lot of damage ups, so they could kill you even without a full charge. (fuller charge does higher damage)