sedatives like the drugs you listed all depress the Central Nervous System and can depress breathing to dangerous levels. Mixing sedatives (including alcohol) greatly increases that risk, and is generallly not considered a good idea. If you must mix drugs, ingest a smaller dos of each drug than you would normally take and resist the urge to redose too soon after ingestion. I wouldn't say it's safe, and why would you want to mix these drugs when one of them alone is sufficent to knock a person out.
Are you going to several doctors? Valium and xanex are prescribed usually for the same thing. I also think soma and flexeril are prescribed for the same thing. Doctors are a good thing but, you better double check them! They are not always right