Student or Learner
If a guy and a girl meet each other, and find themselves in harmony in ideas, ways of thinking, opinions, etc, do you say "They hit it off"?
Is it only for a guy and a girl or two same-sex people as well?
Also, can you use "I and Mike are compatible" or "I am compatible with Mike", right?
What other similar expressions do you have?
Your understanding of "hit it off" is correct.
It can be used between any two human beings, regardless of their sexual orientation.
It's better to say "Mike and I...."
Other expressions include:
We got along well together. Mike and I fell for each other at first sight.