They are usually done that way. I've tested it with two different email accounts and signed up a week apart. Sometimes I'll get a notification on the first and the other never gets a heads up.
The only potential issue is if you went to lighter springs/firing components for a trigger job, you MAY have some ignition issues. As with anything, test and verify first.
However, dimensionally SRP and SPP are externally identical. Large rifle and large pistol are not, as the cup is deeper on...
What projectiles? Your slow twist tells me you will be pretty exclusive to light projectiles. All my stuff is for 105-110gr projectiles, but my barrel twist is a 1:7.5.
There's a great resource online and you should read up on stuff at: https://www.6mmbr.com/6mmbr.html
Cheaper pricing, especially when paying shipping and hazmat. As long as your powder doesn't get wet or have extreme temperature shifts in storage, it's quite stable and will last a long time.
Load to magazine OAL (or just shy) and do development from there. It's what my friend did with his 22 Grendel and it's what I'll do once I get my barrel in. There can be quite a bit of jump in the auto-loaders, but you'd be suprised what non VLD bullets can do with the jump. The 75gr ELD has...
If it's under max OAL you should be good. Unless you've got a very tight chamber, you shouldn"t see too much variation due to length if youve got decent dies or are running a mandrel to help control neck tension.