I have kept a few Amazon Puffers and currently have one in a community tank. Amazon puffers are one of my favorite fish of all time. I will try to answer your questions to the best of my ability/experience.
Amazon Puffers are true freshwater, mine is at roughly 6.8-7.0 pH and around 200 tds
I always kept mine in a community tanks with a variety of fish. My current Amazon puffer is in a 75 with Neons, Angels, Plecos, Cory’s, guppies, and Nannacara Anomala(peaceful dwarf cichlid. I never observed my Amazons bothering any other fish.
For feeding all my Amazons had different personalities. Vibra bites were the staple for mine. I also had them bite into algae wafers and a variety of different pellets. I believe that keeping them with other fish helps with them taking prepared foods, monkey see, monkey do. I had one in the past that was tong trained with freeze dried krill. I also tong trained that one with meal worms. I learned the hard way with the meal worms. Meal worms bite. That particular puffer got bit in the chin by a meal worm and passed from an infection. Cut the heads off the meal worms before feeding.
6 in a 125 should be fine. I had a group of 5 in a 65g with no issues. For snails a 10g tank or even a 5g bucket with a sponge filter and heavy feeding will produce plenty of snails. Most “pest snail” explosions are from over feeding. When feeding snails to puffers don’t add too may, I think they kill snails for fun and that could add bio load to the tank. A scud culture is worthwhile with Amazons especially young ones.
As far as water changes go that will depend on the rest of your stocking and filtration. Every tank processes nitrates a little differently. I probably do 50% per month on my 75g.
I don’t think stocking order will matter all that much. Although all my Amazons I had in the past were added to established tanks but new fish were added after the puffers. Definitely QT the puffers. They always come in rough. Look for fish that have “full” bellies. Get them eating good before hitting them with meds.
2 weeks may be OK in an established tank with plants. The puffers should be fine with snails, the other fish depending on what you choose maybe not. I frequently travel for work and my fish are fine. I feed them a little heavy 1-2 days before I leave and again when I get back. I am only gone Mon-Fri.
I hope this helps.