Ricciardo is an extremely good qualifier (for example he managed P6 in Bahrain in a car that was vastly inferior to everything but the backmarkers), whereas Webber often ends up behind slower cars on the grid.
And as far as da Costa goes: I said he MIGHT be better than Vettel. At the very least he's by far the best driver RB has produced after Vettel.
Not always, and also Webber can get pole position at times and can also qualify reasonably well most of the time. da Costa may be very good like you said