The Core Rulebook says that you shouldn`t limit yourself to playing the species in the books. That is very cool! But how do you make your own?

Are there any official or unofficial guidelines for constructing and balancing your own homebrewed species, and what about adversaries?

Order 66 did an episode on NPC creation. I put together the show notes into a PDF in a thread here. That doesn't cover new species. There are quite a few threads on various specific species that people have created stats for.

No official guidance or formula.

You have to build them to suit the encounter. Typically though you can usually find what you're looking for in the Adversaries Chapter of the core books. If not, you can usually find something pretty close and adjust it.

The big thing is to focus on roles and functions and build to that. Don't go overboard, you want something that's easy to read and apply. A lengthy skill list, Talents and abilities up the wazoo, and half the Galladinium's Catalog for an inventory is not easy to read and apply.

Also, unless you've got some specific use planned int he encounter, skip things like knowledge skills. They know what they know and that's what they know. Focus on the skills they need for the encounter, the talents they need for the encounter, and the gear they need for the encounter. If they are a recurring character you can always adjust before their next appearance.

Order 66 did an episode on NPC creation. I put together the show notes into a PDF in a thread here. That doesn't cover new species. There are quite a few threads on various specific species that people have created stats for.

O66 is good, but they did have an error by omission in that episode. Minions do not have to be grouped. It's a common assumption they do, but they don't. Sometimes this might be for tactical reasons (6 Stormtroopers group while a solo trooper flanks) or it might be for fluff (these are just street thugs, they don't know how to work together that well).