I also have a twenty year old inlet. 1-1/2" pvc pipe, no threads. Therefore, the pool skim insert is only a friction fit. I had to use pvc glue to build up the insert (will not be permanent if done underwater. don't use an epoxy). The kids' floats bump the skimmer and occasionally it will loosen and fall out. But I figure thats okay, it is a safe weakpoint. I have thought about tapping my inlet pipe, but this works well enough.
The inserts that come with the pool skim facilitate an off-the-shelf fountain(s), which is good for raising your ph without affecting your alky. And they're a lot of fun.