Can I say " hezká holka " ?
Nope. not for 'good girl' You could say "hodná holka" as in well behaved girl. Hezká is pretty
but "hezký víkend" means nice weekend, so can I read "hezká holka" as nice girl?
The problem is that in English "nice" means "hezký" only when referring to things. When you talk about people it is a desciption of their inner beauty. While "hezká" is external look. Nice girl in Czech would be Milá or příjemná holka.
What's the difference between holka and děvče? Are they interchangeable?
Could this phrase be used when addressing a pet? Or is it strictly meant for humans?
whatever you want