Can I install the SpeedBraces more than 16" apart?
You can do 32" on center, but it really depends on the type of material you are using as the shelf, countertop, etc.. When weight is applied and the material sags between the braces, the SpeedBraces will twist and ultimately will fail. As long as there is no sag between each SpeedBrace, and you install into studs, they will still hold up to 1000 lbs.
Are SpeedBraces sold in pairs or individually?
You always give the load rating as 1,000lbs/pair. Can this be safely restated as 500lbs/brace? So if we’re doing something longer – about 3 feet – and support it on 3 Speedbraces, would it hold 1,500 lbs (evenly distributed)?
I'm installing a floating table that's 21in deep. Which FastCap SpeedBrace length do you recommend? Also, is the level cleat needed to be installed along the wall or is this optional?
The level cleat is completely optional. It just makes install faster and easier if doing a lot of them. As far as size, that’s really up to you. Depending on your material and what weight it needs to support you can choose how much you feel comfortable hanging loose past the end of the brace. If nothing, use a 21x24 with the 21 leg coming out. Personally, I’d do a 15x18 for most applications of a 21-inch shelf though and that should work great as long as you don't plan to have excessive weight right on that outside edge. Also, check out the Tech Specs tab. There is a lot of good information there.