Ok, I watched again the American Splatoon Direct:
https://www.youtube.com/watch?v=kQsZPYB0vMw&t=16m50s
The number of abilities each piece of gear can have is based on its star count. The more stars it has, the more slots it will have.
Up to 4 abilities can be equipped per piece of gear.
However,
other than the first one equipped, abilities are assigned at random. Frequent usage of the same gear apparently generates more abilities to equip.
In this way, squids can level up a piece of gear to enhance its strength, or make up for its weaknesses. And so, gear combinations can improve one's strategy.
Spyke can increase the number of slots allotted to gear. That said, one can't make a request without a Super Sea Snail. And it won't be always easy to acquire one. If one does take advantage of the services, it may be possible to optimize the abilities of every single piece of gear.
On-screen Spyke text:
I can reroll abilities on items wif full slots!
It's still not 100% clear if equipped = active, but it is likely to be.