Clownfish are always a winner in my book. Black cap basslets are cool too (purple in color). I am personally not a fan of damsels or chromis. Maybe a dwarf lionfish. I have a 45 gallon also so speaking from experience.
The six line has a reputation for being aggressive. Puffers are not reef safe. They also have strong teeth and jaws so they can bite chunks out of other fish. I understand that they are not that aggressive but if they get pushed.......
I would suggest that you go to the Live Aquaria site and check up on fish that are for nano tanks, are easy, peaceful and reef safe.