In other words, the card backs of the Power Up evolution cards are the same in editions one and two (and thus either edition of Halloween's evolution cards are comparable with either edition of Power Up).
For what it's worth, they all appear to be the same (looking up images online, not in person).
But cardbacks of Evolutions are the least of worries, as each monster has their own stack anyway > no 'mixing' unless you're playing a variant.
Specifically, I want to mix Second Edition Power Up with First Edition KoT and Halloween, and from what I've seen, looks like I can do this.
Is that correct?
Everything is compatible with everything.
And Power Up! is (currently) the only one where you would definitely want 2nd over 1st.
So yes, you are perfectly fine with what you are doing.