I've come across a conflict as far as the the title sources are concerned:
You may choose a familiar with an alignment up to one step away on each alignment axis (lawful through chaotic, good through evil).
E.G. voidworm, but also Lyrakien...
In order to gain a voidworm as a familiar, a spellcaster must be chaotic neutral, be caster level 7th, and have the Improved Familiar feat.
Improved familiar states that different alignments can be taken. But the specific entries in Bestiary 2 state in no uncertain terms that alignments must match.
Now which do we go by? My gut feeling is by the newer rules in Bestiary 2.
Maybe some creatures have stricter requirements than others in order to take them as familiars?
I don't see a conflict. The feat states that you can choose a creature up to one step away from your alignment. That's the general rule. Some of the creatures in Bestiary 2 say the caster must have the same alignment as the creature. Those are specific exceptions to the general rule. At least the way I see it.