What's the recommended spacing for securing pex to studs? Do you think it's OK to leave long stretches (8-10') just laying between floors?
Also from a previous professional plumber job I have these inserts that hold the pex where they go through the floor. I could not find them anywhere so I just drilled a hole slightly larger and might put some spray foam around them although they will probably still touch some wood. Is that OK?
Does PEX not need water hammer arrestors since it's plastic? I didn't see any on the previous pro job.
If you have access you need to secure it about every 48". If you're fishing it through walls then there's no way to do that.
PEX touching wood is not an issue. Foaming will work just fine if you want to.
Water hammer isn't such a big deal with pipes that can flex like PEX does. You still need arrestors at fast acting valves in my opinion...Dishwasher and washing machine, but I wouldn't bother putting them anywhere else.