If you are planning to plum in rigid PVC pipe then they make bulkheads that are slip x slip connections like this:
http://www.bulkreefsupply.com/bulkhead-abs-slip-x-slip-1-7c63a554c36ea63c77723a472b7ca20f.html
Notice that the threads are still present on the dry side of the bulkhead, that is so you can tighten the nut that holds the bulkhead tight against the glass. You don't want to ever try to thread a piece of PVC to those outside threads. When people are talking about threaded bulkheads, you would be able to feel the threads inside the bulkhead.
If you look at the pictures of bulkheads in that link above you will find a diagram with an "A" measurement, a "B" measurement, and a "C" measurement, I believe that "C" measurement is referring to the diameter of the physical hole in the glass. You want to be pretty precise with your measurements because your 1.25 inch measurement puts you between a 1/2 inch bulkhead and a 3/4 inch bulkhead, a 3/4 inch bulkhead has a diameter of 1.33 inches.
There are also thread x thread bulkheads, thread x slip bulkheads, slip x thread, and I believe there are bulkheads with male barbed fittings so one could connect flexible tubing.
Watch this:
http://www.youtube.com/watch?v=P64AhetQf2c It's a plumbing video from Vivid Aquariums, very good info on aquarium plumbing!
