It is pretty hard for a "novice" and evensometimes for a vet or rabbit breeder to tell the sex of a bunny,especially one at six weeks. You could try but it is a tricky business.I wouldn't take the risk of keeping them all together, especially ifyou are not 100% sure of the sex of the new one. If it is a male in afew months you will start to see the testicles drop that is onesurefire way of determining if it is a male/female. You could also finda rabbit savvy vet and have them sex the bunny for you.
Fergi's mom