Is it okay to take plain benadryl (diphenhydramine) with an anti-depressant?

Yes: Do not combine it with an maoi, but these are rarely used these days. There should be no contraindication to combining the two. Just realize that you are taking a sedative and that some antidepressants are already sedatives. Benadryl (diphenhydramine) also impairs driving the next day, so be very careful if you need to be on the road and you are still groggy from it. Avoid long-term use: stops working and causes^wt.