PEX to copper

I've had two underfloor leaks (copper piping under and through slab on grade concrete floor) and now plan to re-plumb most of my house with PEX. I'm very sure the leaks are the result of poorly protected copper in contact with concrete which has pitted and finally caused pin holes in the copper. My plan is to abandon all under floor copper and use PEX fed through the attic. Some of the cold and most of the hot water is above the floor, so I want to connect PEX to copper in those instances. I'm not a plumber and I've never used PEX so I'm at ground zero. Any guidance would be appreciated !

Copper to pex transition fittings are readily available. Most solder onto the copper, then have the appropriate fitting to either crimp or otherwise attach the pex to it. There are some push-on fittings, but you're better off with the transition ones. For temporary capping things off, you might want to pick up a couple Sharkbite caps. They'll seal things when you get to a stopping point, but can be removed fairly easily.

If your attic temperature drops to freezing or below, you will have problems. As I recall, Austin does get below freezing at times.

While the Pex may not split as easily as copper, you will still have frozen pipes, and they may split.

If you don't run it near an outside wall and run it right on top of the ceiling with all of the insulation over it, it won't be much colder than your ceiling. But, if it gets uncovered or there's air circulation or it gets next to an outside wall too close, yes it will likely freeze. On the other hand, in the summer, it will likely be as hot as your ceiling, which could make the cold rather warm!