If i had one alcoholic beverage at 1pm, will i be okay to take benadryl (diphenhydramine) for my allergies at 7:30pm that same day?

For your body, Yes.: Certainly for your body it would be. (Assuming that this alcoholic drink is of average content (no more than 4-5 oz. of 40% etoh content). However, driving after this may not be and I would not recommend it until the following morning.

It is OK: In general there is a risk but in this case it is low. You need to discuss this with your physician because I have little data about you to given you much advice