Can you mix a Sandfire Bearded Dragon with a common one The common one is a little older and the Sandfire one is around a week or two old?

I would advise not to. It depends on the gender and sise. If both female or one male and one female then that's ok. If the other dragon is half the sise of the other then it will cause the bigger to be 2 doniment and cause huge stress on the other. Its up 2 u just keep watch cafully and give them space. About a 150 gallon should be enough! =)