Here are a bunch of other thoughts.
1. Mammoth is a very good program {big, and good}. I would absolutely get the advice of whoever the equipment "guru" coach is for the U12 age group. Believe me, there will be one who knows VOLUMES more about this than most others. And he/she will want to be helpful. It's also "not good" if you ignore coach input and then blow it.
2. I have three kids who have coached kids at the U12-FIS Levels {still do} at some of the biggest clubs/academies in the country. The hardest parents to work with are the ones who feel that they are experienced in this. One of my kids had a U14 group where
every kid had at least one parent who had skied on the WC and they there the easiest group of parents that he every dealt with, as they followed his recommendations {and at that age let him work directly with all of the companies}.
3. Sounds like quite the stockpile of boots! I am sure that you know how to fit boots, and even deal with he chicken legs. But let me askjk this. How do you know that they are the best/right boots for him? Not size, the actual boot design. There is a best boot for every kid, at every age. And they can vary by age. One of the clubs that I mention had a LOT of U14's in an Atomic boot, yet none in that boot when they kids moved into their first plug boots. So, it's worth a conversation with the coach{es}. I have a daughter who is 30 months younger than our son. She never used any of his "stuff." He was pretty hard on it, and in terms of boots, their foot shapes, and basics physiology was entirely different. Actually she was quite small but git her good sized adult foot early, which posed real problems.
4. Ski age. I would be pretty hesitant to hold onto skis for three years after your son has used them, and pass them on to your daughters. I might take a chance if they were new, properly stored, etc. But, the technology on even their skis does change every couple of years. My experience with girls is that right about at you r daughters' ages, if in a race program, fitting in is important. And that even extends to having skis like a lot of the other kids. When they are MUCH older, used older speed skis can be cool. Otherwise.....Just think about it.
5. There is absolutely no real reason to buy the same brand for GS and SL. 98% of the ski parent world does it. Heck.....we're used to it. If you're dealing directly with a race rep {not some local "rep"} and are concerned with a relationship and getting good USSA pricing, it makes all the sense in the world to have the brand be the same. In fact, with three kids, it might be even ore advantageous. Ask the coach. In terms of product? My concern when ,my kids were younger was to get them on the best equipment for them. In fact, for about a four year period, the best SL skis for the were not the same brand as the best GS skis, and the best boots were not affiliated with either company. And by the way, that was four different ski companies and two different boot companies. We had plenty of funny conversations. Really funny when that dreaded sponsorship word comes out of other kids and parents mouths. That could be a whole separate and very humorous thread. Believe me.
For example, when my daughter was a J4, Elan made a tremendous SL ski and a horrific GS ski. Volkl made a great GS ski, and a not so great SL ski. Make sense? You'll probably end up with one, and that's fine, too......as they are all pretty darn great.
6. Mammoth tends to have good snow. If you were in the East, where the snow can "vary", I might have different ideas about how many pairs of skis, and trainers versus race skis. If you have the time and interest to keep the skis in really good shape, it sounds like you'd be fine with one pair of GS and one pair of SL skis.
7. If I were back in those ages, I would have my kids ski ever possible minute on their GS skis. Some very, very god coaches have a bias for SL skis. Including the guy who coaches MS as a J4 and J3. But as
@hbear mentioned, knowing how to ski a GS ski, and how to work it, how to truly pressure it, how to really carve it with good balance is all key. I would be dollars to donuts that your sons's coach will be more concerned with that than anything else next year. Forget the gates, forget the clock, forget the results. Making good, clean turns . So back to me "ask the coaches about boots and skis......" About four years ago, I was watching a coach working with a group of first year U14's. These kids are now amomg some of he best first year FIS skiers in the world. Their training venue was sprinkled with courses, and kids, and a lot of slapping practice. His kids were all free skiing, with serious purpose. No gates. I watched them ski down and when the other kids were headed in, get back on the lift. The eventually come down again. This time making what looked like perfect GS turns in succession. So I asked him what he was doing. "Trying to get them so that they can make 4 good turns in a row. We're close. When they get to about six or eight, we'll train in a course a bit more. They are never going to reach their potential until they ski exceptionally well." Funny, this past week we heard that about Hirscher, HK and MS among others. Very different than most clubs.
8. If Blizz and Nordica kid skis are not twins, they they are close siblings. I'll ask. Or.......ask the coach. And I'm not saying that I wouldn't buy a 2014 ski, but some of these skis are a lot better. Others were already really great and have changed very little. I believe that Fischer has made very few changes, and that's still a great ski. Rossi has had a lot of changes, and thee are a lot of coaches {and kids} who love their line up.
To be honest, they are almost all good. I know you have boot experience. But I'd want that right, first. I had my son in too much boot, too soon, one summer. And my daughter in not enough boot. They fit perfectly.
So neat tat you son loves this. Our youngest is 28. We love to ski together as a family, when we can. 3300 miles of distance make sit hard! Enjoy these days and have fun!!