How do you know when to change grit size?
In a vibratory tumbler you start out with medium (220) grit. not course (100), then go to 600 grit then polish.  If you ever buy a grit kit. make sure it's for vibratory, no 100 grit.  You can add a step and go to 320 or 400 before 600.  It saves a little time, not much.

The way you know when to chance is when you've gotten them as smooth as you want them to be.  That means remove pockets, chips. etc.  If most of the pieces you are tumbling look good but there are just a few that need more work, go ahead and switch to the next grit.  Run the ones that need more work through the next batch to finish them.  You want the stones to be pretty good before switching from the 220 grit or they'll take forever in the 600.  This is the long step.

My two vibratory tumblers usually take about 3-4 days on 220, 12 hours on 400, 1 to 1 1/2 days on 600 & 1 day (chance polish at 12 hours) for the polish.  I change the grit/polish every 12 hours, washing the muck out into a 5 gallon bucket to settle/evaporate.  You can pour off the clear water on top after it sets several days.  You don't want to dump the muck down the drain unless you like plumbers.  The rest will evaporate and can be put in the trash.

Can you save the grit and use it again for the next batch of rock?
If you are doing it right. there won't be much grit left to save.  The grit should break down as you tumble the material.  I always had a little grit left on the 220 run that just didn't go away until I paid a little extra for a graded 220 from Ebersole's in Wichita, KS.  It's about 50 cent a pound more than ungraded and worth it for me.

Other vibratory tumblers may not have this same problem.  You'll find out, I buy 5 pound boxes which do quite a few loads and only pay about $3/pound.  Not worth saving any doesn't break down.  If you try to use course (100 grit), it will sink to the bottom and stay there in just about every vibratory I've seen.

Why do they recommend that a separate barrel be used for polishing?
Contamination from the grit.  Somehow you almost never get it all, no matter how hard you try washing it out.  Contamination between grit size changes isn't as crucial but try to wash your stones & barrel well anyway.

Do you put polyethylene pellets in the final polishing stage with a vibratory tumbler?
You can.  Always change the pellets between different grit sizes.  Re-use the pellets only with the same grit size.  The grit becomes imbedded in the pellets and, again, you get contamination.  I bought a bag of pellets several years ago.  Before I started to use it.  I mentioned to a dealer friend that I'd bought them.  He went out in his garage &, came back with a gallon milk jug full of 1/4 to 1/2 in pieces of agate screened from the good stuff.

I have yet to open the, bag of pellets.  You want small stuff to help the tumbling, as long as you have that, you don't need pellets.  I've added to the gallon jug and my wife sifts through & takes out stuff to make things like gem trees if I let her near it.  It gives you a reason not to throw away the small pieces when you are out hunting.  You may never need the pellets.  The chips don't have to be changes between grits.  One caution, you should always tumble stuff of like hard-ness.  Don't tumble obsidian with agate, etc.  The most common thing people tumble are quartz-based.  Agate. jasper, quartz, etc. which are all pretty much the same hardness.

 Editor's Note: If you don't where to get a tumbler or grit, try your local Rock Shop, local Lapidary Suppy Store, Local Gem & Mineral Club (click to search for club in your area) or mail order Rio Grande