We had gerbils when i was a kid, and ended up with s male and female and no real idea how to take care of them. At some point I think we had 30 gerbils in an aquarium- do not put 30 gerbils in an aquarium! (I was 9 at the time- so please do not judge me for being a bad gerbil mom) MAKE SURE THEY ARE BOTH BOYS or GIRLS. Also if they get out of the cage the other gerbils will kill it upon return because it will smell wrong.
And Jack Russels and gerbils do not mix well either. I mean they would have been nice I suppose but be aware if you pull their tails they will come off! (Natural defense) My brother did that to one of them, but then we could tell them apart. Also they love to chew, we actually would let them make most of their own bedding by chewing up old toliet paper rolls.