[glampka]

Connecting rocks together is a good idea - makes your aquascape more stable. There are a number of ways of doing it - gluing them with epoxy, drill holes in the rocks & insert plastic pins , some even make shelves to support the rock.
It is a very good idea to not stack the rock against the glass. It makes a number of things next to impossible like: cleaning the glass & finding dead fish so you can remove them. Since you may not be able to get circulation behind the rock it then becomes a great spot for detritus to collect.