Dude,
The short answer is that some changes are temporary (or only apply to starting units) and some are ongoing throughout the game. The variation in unit sizes applies to:
1. Starting units: I suspect Tyberius did this to prevent certain factions (*ahem* Lithuanians) from getting wiped out in the first 2 turns, and to give some traditionally-passive factions a kickstart (eg: Volga-Bulgarians)
2. Standard unit sizes: Some knight-style units have been reduced from 40 to 30 (Crusader-type knights). Steppe Light Cavalry have been increased to 60!! Some "nationalist"-type infantry units have been increased to 80 or 100, which results in very big sword and axe units!
As for your last question, I'm not sure if there are any significantly game-altering additions, bro. The Scots get some really good pikes (XL-style pikes, not cr@ppy vanilla MTW pikes) which makes them a force to be reckoned with. The Papacy get Swiss Guard, which are another stand-out unit. Anyone who owns Saxony gets access to Saxon Warriors, which are handy, although they ain't quite Huscarles.
Basque warriors are a big addition. They are cheap nationalist-type peasant units who are as good as Ghazis. Deadly for the Spanish.
Elite Bedoins are another unit worthy of mention. They are pure death for knights, even out of the desert. I don't think the Almos get them, though, unless they conquer the usual Bedouin-producing provinces.
Roark.
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