My family runs a store fixtures business, so i consider myself somewhat of an expert on slatwall.
First, there are different types of slatwall. The 3" spacing is pretty standard though and you should be able to find parts pretty easily.
As for the metal inserts, the slatwall must be manufactured specifically for the inserts. The inserts do add a lot of strength, but they make the slatwall SIGNIFICANTLY more expensive. Also, consider that the standard slatwall is pretty strong and will hold up pretty well under most ordinary circumstances (i.e. hanging tools, holding shelves for light items/boxes, etc...)
You can get laminate inserts for standard slatwall to make the inside of the slats match the finish on the face of the slatwall, but this is a purely cosmetic application.
Finally, you would be amazed at some of the specialty type slatwall accessories you can find if you look around. If you think you need to hang it, somebody probably makes a slatwall hook, bracket, shelf, hanger, etc... to hang it on. My suggestion is to call a local store fixtures company and ask. Most store fixtures companies are happy to sell in small quantities and are typically very well versed in what is available and where to get it.