Attaching a smaller rock to a larger base piece is not much different that attaching a frag
. It really depends on the weight of the smaller piece and the angle to be mounted. With all the sediment and such that collects on rock, be sure you clear that off using a powerhead or turkey baster.
The epoxy alone may work well enough if the frag
piece is small enough but you will have much less problems if you use the superglue gel as well. If you attempt just the epoxy route, you won't harm anything except to waste a small amount so no real loss in trying. Just be careful if this is an SPS
as it can be easily damaged if it falls.