Welcome to the forum mate
Tureuki answered most of your questions but I'll make a few additions.
With the Sicillian factions, they will definetly be included. But they will not have brand new rosters. They will have a mix of vanilla and borrowed units from other mods. Unfortunately we don't have the manpower necessary to do all the factions, as much as we want to.
There will definitely be different tactics for each faction. For Ragusa as you may have seen in the preview they rely a lot on infantry units and are lacking in cavalry and archers. They have great crossbowmen and gunpowder units. They will also have trade bonuses and other such advantages to other factions.
The Duchy of Athens will be a very interesting faction. It will probably have the biggest mix of eastern and western type units. The only other similar faction will be the Knights St John. Athens will combine light troops of the Greeks such as archers and javeliners, with the specialised Almoghavers of the Catalan Company and the heavy Aragonese and Frankish knights. But the specialist and the heavy units will have low recruitment rate so they will need to be used with caution. The local Greek troops will have high replenishment rates so they will still be the staple of the faction. The Knights St John will be the same with low recruitment rate for the actual knights, higher rates for western auxiliaries and even higher for local Greek troops.
We haven't started the work on balancing and stats. So at the moment we don't have a definitive answer on which units will be more powerful or more important. But we aim at as much historical accuracy as possible.