as Felix wrotte they have to be original FSX compilation (like default models) ,you can't not use any of the recently models made by independent designers and add to another plane ,WELL YOU CAN

what you can't is redistribute these mixes...some are already protect it, against it..like some b707 hanging on there made by a couple nuts ::)
see the most difficult part to any designer is not the exterior ,Is the VC ,the VC is quite difficult to make ,and we wouldn't want our work to be in another airplane ..get it?