Simple Q&A D&D 3.5 (by RAW) XXII

[Douglas]

Thank you. The source (DMG) is important here, because that book has no say on the topic (monster races and class levels) except in advice to the DM about the difficulty of incorporating such into their game. The Player's Handbook contains the basic rules for playing the game, and for using base classes. The Monster Manual is the authority for monster races. If any part of the DMG disagrees with the primary source on one of these topics: the primary source is correct, and the DMG is incorrect. So sayeth WotC in their procedures for resolving such disagreements.

Thus rockdeworld's original answer (A 192) is correct, and tuggyne's dispute is based on a secondary source and hence incorrect.

That principle only applies if the two sources actually contradict each other. Unless the Monster Manual states that non-humanoids with 1 hit die cannot trade it for a class level, rather than being silent on the matter, the DMG rule is valid.