Off-spec refers to both talents and role. There's no "I'm a DPS druid so I need all the caster and melee stuff huehue". What role you end up in, you will roll as one. There's no "My actual MS is DPS, but I got in as a Tank role here. So my MS is considered tank". You roll as the role you're assigned in as, no matter if it's MS/OS in "reality".
Some classes/specs being able to need/utilize different types of items doesn't make it right for e.g. Enhancement shaman to need caster gear as they are more of a melee spec.
Armor type in RDF is basic. You're plate wielder, you're not allowed to roll need leather gear. Select greed and communicate after rolls if need be. Or ask for permission to need roll, but make sure all 4 other players agree to it, no answer is not 'yes'.
Armor sets may be usable by multiple classes, but they are primarily always designed for a specific class in mind. Sadly some class specs goes against these designs, e.g. Wildheart set being useless for Feral druid, but on the other hand Shadowcraft is amazing one. Same deal as with different armor types, communicate and ask others if you can roll.