0-Level men can use all weapons and armor and still gain XP. They are equally proficient with all kind of equipment (read: very bad)
The only demihumans in my campaign are the Atlanteans, who use the rules for Zaharans. I generally assume that adult Atlanteans are at least 1st level, but we have had one 0th-level Atlantean in the campaign. Since all Atlanteans are supposed to have a demonic familiar who is obligated to try to teach them to use arcane magic, I didn’t have a problem letting him level up into a fighter-mage class similar to the Elven Spellsword. Of course, he had his tongue ripped out by a zombie on his way to 1st level, so it’s been kind of moot so far.
I have generic 0th level normal men and most 0th level mercs recruited as henchmen level up into Fighters. I let mercenary bowmen, longbowmen, and crossbowmen level up into Explorers, though, based on info from an old draft of Domains at War. Carousers recruited as henchmen can level up into Assassins, Bards, or Thieves. Since a 1st level Assassin or Thief has such poor skills, I don’t think you need special training to get those classes beyond hanging around with shady people. A first level Assassin is just a Thug, after all.
In my Barbarian Conqueror King setting, the only playable non-human species are Lizardmen. With strong fighting (and predatory) instincts, a “level 0” Lizardman - anything from a dinosaur herder or coffee farmer to caravan guard or common Lizard-mercenary - fights and saves as a 1HD creature, i.e. as a Level 1 Fighter. Level 0 Lizardmen, however, are not trained in armour use (as they do possess good natural armour), and if they don any kind of armour except for shields (which they can use), they fight and save as Level 0 Humans.
If they gain Level 1, they may normally choose between Lizardman Hunter (the Hunter’s skills grow from a Lizardman’s predatory instincts) and Lizardman warrior (then gain the warrior’s totemic abilities - based on common Lizardman beliefs). To become Gladiators, they must have been specially bred and trained for that from birth; to become Priests Witch Doctors they need to become acolytes or apprentices of the appropriate class.
Young (up to the age of 10-12), old and infirm Lizardmen, on the other hand, are non-combatants, have 1d4 HP and fight and save as Level 0 Humans.