The Galil mags will lock into a Valmet, but it is a very bad idea to use them, since they feed way too low. You will find that the bullet tips strike the barrel root very low and then bounce up and into the chamber. Some guns, it simply jams the bullet back in the case and stops. Eventually, you are going to have a round that gets the bullet jammed back into the case, AND chambers it, which is going to dramatically increase chamber pressure. It probably would never grenade your gun, but just not a real good idea regardless. They will work in a last ditch effort, but not something I would play with in a nice pre-ban rifle.
You may want to check out the Valmet board at www.valmet-weapons.com