Would it be possible to make it so a spider mine will automatically attack a probe? instead of have to manually do it (if thats possible)?

move a zergling under the probe

Moving a Zergling below the Probe would slow it down. I would just put a location over the mine and have it explode if a Probe walks into the location.

Killing a mine does not do any damage. And the probe would get slowed down if you move the ling every 3rd trigger loop.

Oh and no, there's no way to make a mine attack floating units like workers, archons or vultures.

A little slowdown may not be such a bad thing. You may be able to just have it happen when a mine is nearby, if that doesn't give too much away. Depends on the map of course. More yo move the ling, more the slowdown, right? well just make it minimal then, like above... There's a lot of splash, after all.

Splash kills the probe so do what Viett and Ar3sgice said. But moving a burrowed unit under a unit constantly will drastically slow down the unit. Put a condition, like if a Foe brings 1 unit to a large or medium sized location, move the burrowed zergling under the probe, depending on how exact you want it.

Just move the zergling to the probe when he is no longer under it, so it only moves when he does. This should cause no slowdown at all.

Probes are hovering units, so you have to use the splash of the spidermine to hit them, rather than have it actually target them.