So I've read best welling level is 64 because then monsters are 65 and can drop req lvl 55 items.

Now, if I drop in an item that is req Lvl 55, and I'm not 64, will monsters be level 65 still? And if yes, then this would apply for all lower tiers right?

Thank you!

Monster level is equal to whichever is greater, your level +1 or item level.