In my Gizmo-verse, Cassettes and Targetmasters are just different kinds of MiniCons. MiniCons were factory constructed by the Quints to enhance the primary robots. After the war to liberate Cybertron from the Quints, some of those MiniCons decide they no longer want to be an enhancement and prefer to enhance themselves. That's where I like to think bots like Slamdance would fit in.