So the thing about diphenhydramine is that if you want sedating relaxing effects it's best in lower doses, like 25-75mg. That will give the most pronounced anti-histamine effect, which is responsible for sedation, calming, and allergy relief, etc. If you take too much, it becomes more stimulating, because it also has anticholinergic and SNRI properties, which start to cancel out the sedating/relaxing effect. In low doses you don't really get the anticholinergic and SNRI effects, as diphenhydramine has a higher affinity for histamine receptors. So basically less is more. I find 25-50mg a good dose for sleep.
I assume whoever said it raises heart-rate and is an "awful drug for bringing you down" took too much.
Taking diphenhydramine with a benzo is considered relatively safe, doctors will prescribe them together. It is just going to increase the sedation etc, assuming you're not taking an enormous dose of the benzo I think it should be fine.