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Depends on your start date. If you start 1066, you start as the Duchy of Rashka, and get to create the Kingdom of Serbia on your own, which is a huuuge amount of fun. If you play your cards right, the ERE should splinter, allowing you to prey on their remains: however, your other hand has to be holding off Croatia, and if Hungary declared war... Yeah. You might wanna gang-bang the Pechenegs a little bit. Unhistorical, but a good way to gain in power quickly.

Not so familiar with the other scenarios, sorry.
 
Ull prolly need quite a few reloads as Croatia is very very strong and will attack u all the time, and if that happens there is basically no way to resist.
 
Actually, although croatia will attack you soon into the game I seen no problem there - just offer yourself as a vassal. After that you slowly expand to S and SE. Control 6 of 8 needed provinces, collect 250 cash then break free and create a kingdom.