Gerbils and coconut

Hi
I wondered if it were ok to break open a coconut to give to our gerbils. I
think its ok to give them a shell for their teeth, but is coconut meat too
oily?
Also, my partner is anti me giving them anything but "gerbil" food ie packet
food for gerbils. He kept gerbils for years when he was younger with no
problems and never gave them treats but I'd like to give them some fresh
food occasionally. Is he right or are there some healthy bits I can give
them?
btw, we've had these for about a year and they're glossy shiny eyed and
lovely natures.
Continuing from the coconut question, how about other nuts in shells eg
brazils, walnuts. Can they have these occasionally?