I have two recommendations (and they're both relatively inexpensive as pdfs).
I would recommend Mutants and Death Ray Guns (MDRG). With MDRG, it's very easy to create a variety of creature types. And, while they're officially different systems, the Song of Blades and Heroes system uses the same mechanic, so I've occasionally borrowed skills from that system to expand MDRG options.
Another option I would recommend considering is Blasters and Bulkheads (B&B). B&B is not specifically PA but I like the character build system--it's easy to create all kinds of creature types with different abilities and eqipment.