Might I suggest removing the class checks entirely and simply replacing them with a cleric caster level check?
Scale the bonuses with cleric caster level, regardless of what PRCs were used to obtain them. This is much more intuitive and fair than selecting a subgroup of PRCs.
Similarly, the bard set can scale with the mean of Bard Song and Bard Caster level, so a level in a class that advances both is a +1, and a level in a class that advances only one or the other is 0.5.
I believe such functions are already used in the nature and arcane sets, and for determining Golem level.