Is there a restriction to the faction that you play and skirmish attachment?

For example, can i play and Imperial list, with eWing guards using temporary alliance and then give the wing guards cross training?

Same goes for an mecernary list where i bring in eAlliance rangers using temporary alliance. Can i give hem prey on the weak?

You can take only skirmish upgrades that match your army. (Temporary Alliance does not work on skirmish upgrades.)

You must assign each of your skirmish upgrade attachments to one of the deployment cards in your army (limit one per deployment card, no restriction on affiliation). See skirmish upgrade rules from the FAQ and/or rulesheet from an expansion containing a skirmish upgrade (attachment).

So generally speaking you can put skirmish attachements on deployment cards you bring in via Temporary alliance. Both of the examples you provide (wing guards with cross training and rangers with prey on the weak) are valid.

What you cannot do is bring in attachements or upgrades via temporary alliance. All of your attachements must come from the host faction. For instance you cannot use temporary alliance to bring in Prey on the Weak into your empire list.

Temp alliance only lets you bring in deployment cards , not skirmish upgrades (ever noticed that little text on top right corner that says "skirmish upgrade"?)

You can, however, bring in other faction's deployment cards and slap your own faction's skirmish upgrades on it (which is what I think roman above is saying)

So these are legal:

As Imp, bring in eWing + slap cross training

As Merc, bring in Vader + slap prey on weak

These are illegal:

As Merc, bring in cross training and slap on your eWing

As Imp, bring in prey on weak and slap on Vader

As Imp, bring in Temp. Alliance to bring in Temp Alliance to bring in Rebels

Thanks for the replies and clarification

How do you bring in Imperial Deployment groups into a Merc army??

15 minutes ago, perthling said:

How do you bring in Imperial Deployment groups into a Merc army??

With the elite Jawa Scavenger you can bring two imperial droids per Jawa to a mercenary army. (The example above with Vader is a mercenary army is wrong though.)