Byzantines in MTW2

I've been reading on our historical friends, the Eastern Romans right, and I read somewhere (hopefully my info is correct) that they used these huge artillery devices called a heliopolis (some huge armoured trebuchet/siege engine) as they were pretty big in the seiging department.
So correct me if Im wrong, they have these specialized units:
B. Infantry,
T. Archers,
V. Guards,
Some type of professional spearmen?,
guard archers?,
kataphractoi,
pronoai allagion,
b. lancers,
some latin cavalry group (name?),
bulgar brigands?,
alans cavalry

Please feel free to add to the list. Do any of these guys get any upgrades persay? Do the Byz get any advance units later on?

But what about artillery, are they really stuck with the trebuchets and average bombards? no pike or halberd unit?
Will Steppe Heavy Cal be available to them or any other faction?
Is my heliopolis point historically valid in this era? or was it an age of antiquity siege engine?