In this situaion "are" is used in english
Very complicated... some rules of English would say 'is' should be used as the couple are one unit... but in reality I don't think anyone would say 'is' in this case... the couple 'are' should be used instead.
Actually, I would definitely say "The couple is on the dance floor" because "the couple" is one group, and we'd think of it as a singular noun.
The couple = the pair
why pisto and not chão
"pista de dança" is a fixed expression.