About classes with their own spell list

[Masakan]

Do they follow their own progression or do they stack with whatever spellcasting class you have?

[torrasque666]

Are you referring to taking things like Trapsmith on top of Sorcerer? Or something like Sandshaper? If the former, they do not stack. Trapsmith levels do jack for helping your Sorcerer ones. Only prestige classes that say something along the lines of "+1 to spellcasting class" give any benefit to a previous spellcasting class. They do however, qualify for other​ prestige classes that advance spellcasting.

If the latter, then as long as they say "+1 to spellcasiting" they stack.

[Pluto!]

In many cases, houseruling an option of continuation (like the PF Warmind, 3.0 Hospitaler or 3.5 Ocular Adept) would be more than reasonable, and would probably improve the game.

The exception being something like the Diabolist, which gets advantages balanced around its gimped casting.

[Masakan]

Yeah that's what I thought, THat means you cant go 4 levels of bard and then go celebrant of sharess and expect higher spell levels.