Is it safe to take Benadryl (diphenhydramine) with cymbacort, I have COPD and mild asthma but am having some allergy symptoms right now? I also use rescue inhaler

Yes, but. Benadryl (diphenhydramine) is an excellent antihistamine and works very well. However, the side effects (sedation, visual disturbances, dizziness when standing up to fast can be problematic). There are other antihistamines that have fewer side effects like Claritin which are more likely to not affect other medications you may be using.
It is safe. It is safe to use an antihistamine with the inhaled steroid. Occasional use of a rescue inhaler is safe. If you are needing it on a more frequent basis then check with your doctor. Allergy symptoms often times respond to Benadryl (diphenhydramine) which may help your lung condition also. Check with your doctor if you have any concerns.