RE: System Design ?
The idea of using 1" everywhere is that it makes the job easier, especially if someone unfamilier with the design is helping. It completely elliminates the possibility for errors involving installing the wrong size pipe.
As for glueing individual pieces, that sounds like PVC to me. That's the way I did it. Attually I did use both 1" and 3/4". But I was installing everything myself, and I was using 1" pipe for lawn irrigation and 3/4" for feeding drip irrigation. But I wanted the smaller pipe when possible because I was installing multiple pipes in one trench (a huge risk if anyone of them ever breaks, but that's why I burried them at least 12" deep, used sch40 PVC for latterals, and initially backfilled with sand and/or screened dirt).
Generally speaking, the most common way to work with PVC is purchasing the pipes in 10' lengths and glueing them together on site. And it doesn't matter if it's 3/4" or 1", it takes just as long to glue either one, and it can take a long time. After all, the process requires:
1. Cut the pipe to the length needed. Debur if needed
2. Use primer on pipe and fitting.
3. Wait for primer to dry.
4. Apply glue to pipe and fitting.
5. Insert the pipe into the fitting, twisting as you go.
6. Hold the pipe and fitting together for about 30 seconds (otherwise the wet glue tends to push the pieces back apart).
7. Do it all over again for the pipe on the other side of the fitting.
Sure, when you have a large number of glue joints to make, you can find some efficiencies, like priming several pieces at once so that by the time you prime the last one, the 1st one is dry. But beyond that, it takes a few minutes on every single joint. Those few minutes build up over time.