Yes, you can use both expressions with slight adjustments for clarity:
"On the same page" – This phrase is indeed used to express alignment in opinions, thoughts, or understanding, typically in a collaborative or communicative context. Your sentence works, but it’s more common to say:
"I was on the same page as him about the movie."
This version flows a bit more naturally in English. If you’re discussing alignment in opinions or thoughts, this expression fits well.
"What a buy!" or "What a good buy!" – This can be used when referring to a good acquisition, whether it’s an item or, in this case, a talented new team member. So yes, you can say:
"What a great buy!" or "What a good buy!" when talking about a new player joining your team if they are a valuable addition.
Both phrases sound good and make sense in the contexts you've provided!