Gerbils become fertile roughly about 10 weeks, so best to separate before then but I used to rehome mine at about 6 - 8 weeks so that wasn't an issue.
They are different from Hamsters and don't normally mind you handling the pups from early on, I used to start taming mine as soon as they had fur, and before their eyes are open. When they do open their eyes they get a little jumpy but that's soon worked around if they are familiar with your scent already.
I used to have loads, and never saw a mother kill her pups so its not as strictly hands off as Syrians, they are adorable little things