I already have a tube of Solarez Epoxy for my Webber epoxy board.
But I am trying to fix a normal old fiberglass surfboard (polyester ?).

Do you think I can use it on the old board or it won't work ?
Won't stick or melt the foam ?

Nope I think. Don't think the epoxy will stick to the fiberglass but someone else might add to it.

I've used both in epoxy and fiberglass boards but never used them on each other

Epoxy can be used on any board, polyester resins will eat the foam on most epoxy boards so should be limited to poly boards.