Can those be used for a guitar?

My guess is that the string spacing will be too wide for a guitar--even if you place the bridge pieces directly next to each other.

How wide are these? In other words, what's the narrowest spacing I can get between strings?

According to the drawings, the width is 15.2mm (.6"), I'd add a hair in-between so they don't touch each other. Douglas's response didn't answer your 'width' question, but is a valid point. Getting the strings over the pole pieces is important (blade style pickups are less fussy) as is getting the outer strings the proper distance from the edge and equal spacing in the middle ones.

Is for throuhole body?

No. If you look at the 4th picture, you'll see a drawing that shows how the strings ball-end drops into the hole, then by pulling the string forward the ball end goes backwards (towards the butt end) and engages into a built in holder. Pull the string to the tuner once the ball end in hooked.

How do you ground them?

I always drill very small holes from each bridge piece into the electronics cavity and run separate buss wires to ground/sheild. I drill under the bridge piece so the hole doesn't show once installed. You may need to scrape the finish off the bottom of the saddle to get good contact. Also, drilling from bridge to cavity can sometimes be easy, other times near impossible, depending on the layout/location of the two. If you like easy and don't mind seeing a little foil between bridge pieces, you could lay down some adhesive copper shielding tape under all the pieces and drill one wire hole for ground. Be careful not to break the foil when installing the bridge pieces.