Yeah, dwarves can't be mages. As a side note, I strongly prefer the term "mage" over the overly generic-sounding "magic-user." Dwarves can be fighters, thieves, assassins, or fighter/thieves. I believe UA also opened up the possibility for dwarf clerics too. Only humans, elves, and half-elves can be mages. Gnomes and humans can also be illusionists. And gnomes alone can multiclass as either illusionist/thief or illusionist/fighter.
I am not using OSRIC, although I understand there are a lot of similarities between OSRIC and AD&D. I have read through the OSRIC 2.0, even ran a short campaign for my tabletop group with the rules, but I prefer AD&D.
pneumatik, if you want to play a fighter/mage, you will have to change your race to elf or half-elf, which means you have to re-roll your scores. Although since you already rolled up a ranger, I'd prefer you stick with that unless you've thought it over and would prefer something else.
Re: gray elves. They share all the same traits as high elves. In addition they receive +1 to intelligence. Note, however that gray elves are insular. While I will allow them, I would urge you to consider one of the more common races first. Most of the world's gray elves live within the isolationist elven nation of Celene and disdain having contact with non-elves. If you want to play an elf you will have the most role-playing opportunities with a high elf because they are the most socially acceptable of the elven races. I probably wouldn't allow more than one of the less common subraces in the group at once since that would just be too bizarre.