Well, went out and bought the Prima guide. Not sure how much help it will be for me, as it doesn’t really answer the questions I have, but thought I’d relate some of the unit types for the Romans and my thoughts on them on the off chance some folks who haven’t bought the game yet are still interested in the thread. I’ll start off with just the special units and the infantry types and see if there is any interest before I bother putting in cavalry, missile, ships and artillary.
All of the following is attributed to the Total War Prima guide (www.primagames.com), by Bryan Stratton. Hopefully no copy right problems…I’m not reproducing it here in its entirety. 
Incendiary Pig: One shot weapon used to inspire terror and panic…supposedly useful against elephants. No idea…haven’t used them at all.
War Dogs: Used against cavalry to inspire terror…again, haven’t used them.
Auxilia: Disiplined spearmen, provides support to leginaries in battle. Decent early unit…can’t stand up in the main battle line, but good for flanking. (41 soldiers, attack 5, shield 5, hitpoints 1, charge bonus 5, weapon type light, total defense 16).
Hastati: Heavily armored pre-legion troops. Again, decent early unit. (40 soldiers, attack 7, defense 4, missile attack 11, shield 5, charge bonus 2, hitpoints 1, weapon type heavy, total defense 14).
Peasants: These units suck just like in the other games…never use these troops unless you are truely desparate…and even then, don’t use them.
Town Watch: One step up from peasants…only use for garrison troops in towns that are unlikely to be attacked…ever.
Arcani: Small elite stealth troops suited for ambushes. I never used these, so don’t know how they work in battle. (16 soldiers, attack 12, shield 2, charge bonus 4, hitpoints 2, weapon type light, total defense 15).
Early Legionary Cohort: Armored infantry equiped with a heavy javelin for throwing and a short sword and shield. First really kick ass unit you get. (41 soldiers, attack 9, defense 5, missile attack 13, shield 5, charge bonus 4, hitpoints 1, total defense 17).
Early Legionary First Cohort: Same model as above, but better skills. (62 soldiers, attack 9, defense 5, missile 13, shield 5, charge 4, hp 1, total defense 17).
Legionary Cohort: More defense. (41 soldiers, attack 9, defense 5, missile attack 13, shield 5, charge 4, hp 1, weapon heavy, total defense 22).
Legionary First Cohort: Solid troops…backbone of your army. (62 soldiers, attack 9, defense 5, missile 13, shield 5, charge 4, hp 1, weapon heavy, total defense 22). These troops get the special ‘testudo’ or turtle formation that makes them semi-immune to missile attacks when they are in the formation. They also give bonus’s to near by troops because of the regiments Eagle standard.
Praetorian Cohort: Elite and expensive. (41 soldiers, attack 12, defense 6, missile 16, shield 5, charge 4, hp 1, weapon heavy, total defense 23). These troops get the special ‘testudo’ or turtle formation that makes them semi-immune to missile attacks when they are in the formation.
Principes: Good early second line unit. (41 Soldiers, attack 7, defense 4, missile 11, shield 5, charge 2, hp 1, weapon heavy, total defense 16).
Samnite Gladiators: Good shock troops. (20 soldiers, attack 14, shield 0, charge 4, hp 2, weapon light, total defense 14).
Urban Cohort: Top of the line…the Swiss Armored Pikemen of TW:R. (42 soldiers, attack 14, defense 7, missile 18, shield 5, charge 4, hp 1, weapon heavy, total defense 24). These troops get the special ‘testudo’ or turtle formation that makes them semi-immune to missile attacks when they are in the formation.
Ok, thats it for the ‘melee’ troops, though as you can see most of these troops also get a missile attack before they charge.
-XT