formal informal is a bit coplex thing in Polish:
Let's have first names:
The first form is the most formal
and then the grade of formality is less and less.
Of course you can say Przemysław is formal (this form you can find on ID)
adn the rest is informal. But I can feel a difference.
I would never say to my close friend "wzajemnie..." or "ja również..." because he or she would answer me: You are funny man (-: