Kingmaker AP GG Burwood
Monstrous classes are player classes that make you a particular monster, give you a template, etc. The index can be found here: http://www.minmaxboards.com/index.php?topic=270.0
Most, if not all, are designed for the 3rd edition revised dungeons and dragons system. I will update for pathfinder.
Template classes are designed to apply a template. If the character taking the template has monster class levels template classes function as racial substitution levels for your core monster class.
For determining the benefit of an ability that is based on class level in your monster class they stack. For determining what abilities are gained, they, on a 1 for 1 basis, as levels in your core monster class. Thus, a mind flayer 1, mosnter of legend 1 could NEVER gain the benefits gained at 2nd level of mind flayer. It is treated as having it for what abilities the mind flayer would gain when taking it’s next level, however.
For example: a red dragon 2 that takes one level of titanic creature is a 3rd level character. If that same character then takes a level of red dragon he gains the abilities of a 4th level red dragon, skipping all abilities gained at 3rd level of red dragon.