Classes from the Pathfinder Core Rulebook, Advanced Player’s Guide, Psionics Unleashed, Ultimate Combat, and Ultimate Magic are all permissible in the game. Other classes, be they from D&D 3.5 Material or other Pathfinder supplements, may be okayed at the DM’s discretion.
Planar Substitution Levels
Substitution levels are levels of a core class that you take to gain certain benefits in place of the level benefits that class would normally receive. Taking a substitution level is not multiclassing – you remain within the class for which the substitution level is targeted. You can’t take a substitution level and apply it to another class for which the substitution level is not targeted.
Every core class has a number of planar substitution levels, each of which you may select at a specified class level. When you take a substitution level for your class at a given level, you give up the benefits gained at that level for the standard class, and you get the substitution level benefits instead. You can’t go back and gain the benefits for the level you swapped out but – when you take your next level in the standard class, you gain the next higher level as if you had gained the previous level normally.
A character need not take all the substitution levels provided for a class, and a character with an Archetype from the Pathfinder APG, UC, or UM may still take planar substitution levels to replace a level ability they replaced the level ability the substitution level targets with.
These Substitution Levels are 3.5 material, and I do not feel inclined to convert them to Pathfinder if they will not see use. If you are interested in taking Planar Substitution Levels and believe the existing ability for your class/level is underpowered for Pathfinder, suggest a conversion and we’ll talk about it.