I have a moss 590 thats a bit long. I have never had a synth stock on any of my guns before. Can you cut these down? Can I do it myself or do I need a smithy?
I have also seen a technique where you cut a wood insert that fits inside the shortened stock and fix it in place with roll pins. Then you screw the recoil pad to the wood insert.
Depending on your definition of "a bit long", you might be able to just sand it some. Check where your screw holes and if they run more than what you are cutting off, you'll need an improvisation as mentioned above. I would imagine if you cut the wood exact enough, some wonder glue would hold it in place
Try removing the recoil pad and take a look inside the stock. Chances are once you cut it down 1/2" or so you'll loose the molded in tabs that the recoil pad screws into. The 590's and 500's share most parts, so I'm sure you can find a youth-sized synthetic stock for it.
If you decide to try gluing your recoil pad back on after shortening the stock, try E6000 glue. That stuff is amazing, and stays flexible for decades.
I wanted to set up a 20-gauge for my boy to shoot. Started with a 1300 that had the barrel cut down to 19-inches. I marked a factory synthetic stock using a ruler all the way around and then used a circular saw to cut it off. There was plenty of room to put the screws back in the tabs at that point. I went ahead and filled the void inside the stock with lead wheel weights to put the center of gravity rearward and to soak up a little recoil. Seems to work fine!