This is the standard (and very effective) way to connect together individual kitchen units. But do I take it from the OP's description that these units are backless? It's normal to have a back on units like this because this provides bracing to prevent it going into a parallelogram, if you see what I mean. So that should also stop bowing, at least at the back.
How much is this bow? 18mm ply does not normally bow very much. If it is just a millimetre or two then these bolts should sort it out fine.