I would make sure they are safe to put in your tank. Try running the rocks in hot water and scrubbing them down. I heard that boiling rocks can sometimes make them pop. So I don't know if I would try that. Some rocks can mess with your PH, if you want to be safe Place the rocks in a bucket and check the PH daily to see if the rocks affect the water too much.
Slate is fine. You can use the vinegar test on rocks. Drip some white vinegar on them, if they fizz they'll raise your ph, if not they won't. I give my rocks a peroxide bath then pour boiling water in them and let them sit for about 10 minutes.
What kind of plant were you thinking of? If you go with anubias or java fern you can tie/rubber band/glue the rhizome to the rock. You can either leave the roots free floating or plant them, if you plant them just make sure the rhizome is above ground or it will rot. I leave mine free floating personally.