Are there any negative side effects from taking Tagamet while taking diazepam? I've heard conflicting remarks.

Consult with your doctor of pharmacist regarding medications. Conflicting remarks mean nothing unless they are from medical professionals.

And it isn't just interactions known to pharmacists and doctors. It's the overall health of the one taking them.
There is no drug on earth that isn't totally harmless, not even aspirin. They are all a tradeoff, a matter of need vs side effects.
Not all interactions are known or well studied, and not all of any individual's health condition are always known either. If you had the early stages of kidney or liver problems, for instance, and didn't know it yet, there are countless drugs that can make that worse.
So there's no reason to be asking this without ongoing contact with your doctor about what you are experiencing, even if you get the OK to take any drug or combination of them.