I can see why you would say this, but I think that's something better handled via inventions or doctrine choices. Legionaries and phalanx are both clearly "heavy infantry." The differences are in specific equipment and tactics.
About archers, slingers, and skirmishers: I think one of the reasons I wanted them to be separate units is that specific areas were famous for specific types of units. For example, Rhodian and Balearic slingers, Cretan and Scythian archers, and Thracian peltasts. It seems to me the whole concept of trading a certain good to an area in order to make a troop type available needs rethinking.
Also - not to get into naval units too deeply, since that's a Pandora's box of its own - I was reading about the ships used by the Gauls of the channel coast. They used big, oak-wood ships with leather sails suitable for the weather of the Atlantic coast and the North Sea - very different from galleys and trading ships used in the Mediterranean.