Hi Sweetlilly,
Yes you can make them from isomalt. I prefer silicone molds to plastic ones. The best way to store the gems is in an air tight container with a silica gel pack in it. The side that was in the mold will be a little cloudy but the other side will be clear. You can make the gems as early as you like just as long at you keep them in a container.
You can use edible glue to put them on the side of the cake but I just tried piping gel and I found that more effective.
Hopefully I have answered all your questions.
