Screeching Women, I want to see what Cataphracts do to them.
SPLATT. Actually, I modded out Screeching Women for a different reason...Germania makes them in droves because they're cheap, but with a defense of 1 they don't auto-calc worth a crap (not even to mention the silliness)

Can I mod in British archers?
Yes...I did the same for Carthage. Seems rather foolish that a basic archer unit wasn't available to them, either. You can cut/paste the vanilla stats from another faction, but you have to enable the faction to have them and you have to mod in a unit card to represent them, as well.

Heavy Onagers vs Onagers. Are the Heavies realistic, do you know?
Probably. This person makes a good case for the onager not having wheels, having been purpose-built on site:

http://www.mywizards.com/catapults/onager/

Want to build your very own?.....

http://www.algobeautytreb.com/