Hello
Bubblezbob, the plastic warlord and wargames foundry metal ones can be mixed because they are about the same size.
For battered shields you can always scratch a little on their shields. Its not dificult to do it.
For more variety on head type you can always buy separate heads from other manufacturers and glue them on the romans. Its what i do to have variety.
If you are going to build an army, invariably you will need to use a lot of plastic wich is easyer to convert/transform than metal and less expensive.
Cheers
Rumacara