Where are the rules for making or modifying these adversaries?

What are the limits to a minion group (I saw someone posted here "max count of 6")?

How do you scale the threat these NPCs pose against the PCs?

Does XP factor into this in some way? Why not?

1) There are no rules. Make them as you see fit. Cherry-pick talents from anywhere, and make the Characteristics, skills, and other stats as you desire. NPC adversaries have no Career and no Specializations. Just go with what looks right to you.

My advice, however, would be to not bloat them with a bunch of talents. It's hard to remember what each talent does in the middle of an encounter, and the GM looking up different abilities for each of his NPCs can really disrupt the flow of the game and slow things down.

Further advice: Start simple, maybe by modifying what's in the CRB and going from there. Add the Adversary talent if you want them to be harder to take down.

2) No limits. But more than 6 in a group (5 skill upgrades) would only add to the Wound Threshold total, since skills are capped at rank 5. It's really up to the GM how many minions should be in a single group.

3) Here's a rule of thumb:

If you want them to be easy to take down, have them roll (in general) one less die than your PCs.

If you want them to only rival the PCs, have the dice pools for various skills be roughly equivalent.

If you want them to feel like a real challenge, have them roll (in general) one or two dice more than your PCs.

Also, if scaling up: apply liberal use of the Adversary talent and increase Soak values, Wound/Strain Thresholds, and Defense rating. These can be applied and increased at the GM's whim. There's a talent for each of those, and as I mentioned before, they can be "cherry picked."

4) No, because this game doesn't have a "CL" or "CR" system. It's just the nature of the game. When there's no levels and nothing even resembling a "base attack bonus," factoring XP into an NPC's build is meaningless.

There aren't any really, trick em out as you see fit to provide a challenge and fun to your players. There were some xp advancement rules for Nemeses in the EoE screen.

There aren't any limits to a minion group's numbers, but I cap the dice pool at what would represent a 5 skill and a 6 attribute, additional minions beyond that are an ablative layer imo.

Generally unless your PCs can take some hits and are packing serious heat you typically don't want more target sets than attacks they can get off in a single round. Depending on the composition of the enemy that can even be challenging if the opposing force can soak up damage.

I'm not aware of any simple xp to xp comparison. Given the differences in combat capacity of each spec it's probably too complicated to bother trying to determine that way.

If you create them using the GM Tools of my character generator, I calculate something I call a Power Level that can very roughly be compared with character XP to give you a ballpark estimate on how tough they'll be. it's not an official value or anything, just an aid to help with adversary creation.

Unless you're fighting a nemesis, an easy way to scale an encounter is to just adjust the number of adversaries in a group.

Of course straight xp won't help that much either, you'd need to look at how much has been spent on things that are combat relevant. A 10 xp Commando is still going to wipe the floor with a 200 xp Diplomat who never bought any combat skills and still has his species base brawn and agility. And if that Diplomat is in a team with another 200 xp commando then a good challenge for the latter will murder the former. That's the thing with classless and class light systems, the freedom is wonderful as a player but it can be a tricky balancing act as a GM.