Advice on Roman vs Roman tactics?

[Somebody Else]

As you're playing against armies that will closely resemble your own, apart from bringing the best in each category of unit to the field, the main onus is on your own generalship. If you bring the same army as what the AI brings, outmanoeuvre them on the field, and that should be that. As far as I can say, there's no such thing as an ideal 20 unit army - as a different composition would be better suited to each different occasion. Just make sure you know what you're up against, and kit yourself out appropiately.

*edit*

Oh yeah, almost forgot. Just bribe them, then you have not only removed their army, but increased your own.