Dwarf Class Options:
- Single-Class Fighter (max level 6)
- Single-Class Cleric (max level 6)
- Dual-Classed Fighter/Cleric (max level 5/5)
- Upon reaching level 5 in a single class, switching to dual-classed is prohibited
Elf Class Options:
- Single-Class Fighter (max. level 7)
- Single-Class Magic-User (max. level 9)
- Dual-Class Fighter/Magic-User (max. level 4/8)
- Upon reaching level 5 (fighter) or level 9 (magic-user) switching to dual-classed is prohibited
Halfling Class Options:
- Single-Class Fighter (max. level 4)
- Dual-Class Fighter/Magic-User (max. level 3/2)
- Dual-Class Fighter/Cleric (max. level 3/3)
- Upon reaching level 4 (fighter) switching to dual-classed is prohibited.
Dual-Classing: Once a character becomes dual-classed, he is limited to the armor of the more restrictive class at all times. However, apart from the armor restriction, the character may use the better weapons and statistics of the two classes in all other cases. The player need not "declare" which type of character he is adventuring as in a given session. The XP earned are divided between the two classes as the player sees fit.
This way, every race has three class options of some sort. Basically, the new level limits come from the XP required to reach the existing level limits in the rules as written, just redistributed. The one exception is for the elven single-class magic-user, where the XP of a fourth-level fighter added to that of an eighth-level magic-user still only leaves the magic-user at eighth level. So that one I'd bump up to level 9 to justify single-classing.