May i use this flush mount conduit clamp side mount horizontally?
You could try those like that if you want.

We use pipe hangers Called "Minnies" in the trade since a company called Minerallac makes them.

They look like this.

The hanger you show is a Caddy Clip and is made to drive on to the edge of a steel bar joist. It will work but they are actually kind of expensive for the same purpose.

You can use the Minnies to run parallel or perpendicular to a floor joist.

And yes if your conduit is less then six feet one in the middle will satisfy Code.

Good luck!