Yes, you only get the role bonus (extra gil and seals) if you queue as the specified role. And yes, the role that gets the bonus is adjusted based on whatever is most needed in that particular roulette category at the time.

The tomes are part of the base daily reward, though. You get those regardless of your role, as long as you queue as a 50 and it's your first roulette in that category for the day. AFAIK, the bonus is only gil and seals. Or was it gil and XP? Either way, it's nothing huge.