There is a maximum number of 36 races in the game, therefore the possible race IDs go from 0-35 (inclusive).
The race IDs are as follows:
Major Races - There is a maximum of 13 major races and their race IDs must be between 0-12 inclusive.
Minor races that might appear at the beginning of the game:
19 Dark Yor
Special Minor Races - might appear due to random events, etc. Apart from the Dread Lords and Pirates, these minor race IDs could probably be overwritten
23 Dread Lords (reserved)
26 Pirates (reserved)
29 Monster (unused)
30-35 are unreserved.
If a race config's CivType field is set to 3, it will be added to the list of minor races and can potentially show up at the start of the game. The CivType for Major races is 4. The Dread Lords, Pirates, and civilizations with the CivType of 5 (special) will not show up in the minor races screen. All others should be fair game, and if they're not showing up, it's probably a bug.
I think that I have more information about modding the civ types in my modidng guide which is linked here: