Is it legally permissible for a contractor to refuse to go along with an architect's instruction which necessitates the acceptance of a quotation performed by a sub-contractor who has been chosen by the architect?

Generally speaking, it is the case that if the firm or individual in question is not named in the contract documents that the contractor has the legal right to refuse to accept the situation given in the question. The contractor may have to be able to show why the refusal was reasonable to a court.