Can an Imperial squad use its version of "Temporary Alliance" to bring in a Scum figure and the Scum version of "Temporary Alliance" to bring in Rebel troops as well.

Potentially, this means that you could have Imperial, Scum, and Rebels all in the same squad!

Any rule against this? Cards in question shown below:

Yes, there are rules to prevent it.

"Some Deployment cards have the Skirmish Upgrade trait. Rules
and abilities that reference Deployment cards do not affect, do not
include, and cannot manipulate Skirmish Upgrade cards unless
the rule or ability states otherwise."

Temporary Alliance cannot be used to include Skirmish Upgrades from a different affiliation.

Yes, there are rules to prevent it.

"Some Deployment cards have the Skirmish Upgrade trait. Rules

and abilities that reference Deployment cards do not affect, do not

include, and cannot manipulate Skirmish Upgrade cards unless

the rule or ability states otherwise."

Temporary Alliance cannot be used to include Skirmish Upgrades from a different affiliation.

Good to know! Thanks! Didn't catch that.

Wow, I didn't know that either. What stopped me from doing this was the fact that Temporary Alliance is unique.

Same reason you can't grab a Bantha and Beast Tamer in an imperial list.
You can have a Bantha.... but not the skirmish upgrade.