There are cement glues that could be used with the small sizes. They sound like river rocks. If they are smooth you can't go too high because of stability, maybe 3 stones high starting with more of the larger rocks as a base and go up from there.It is not a big issue with this small a wall but have a very slight inward lean towards the dirt and not perfectly straight. This helps prevent the wall from toppling over. There is also cement caulking in a tube that you can use to fill in between to give a more finished. Go slow with the caulking and try a mortor trowel.Some combine the trowel on the end. There are lots of things in the building stores that can be used for many "off label" applications. I always think of what it is I want to do then go look in the isle to see what is there. Good luck. It will look nice.