Welcome to the board VengefulPickle

It is very easy to change the game to play any one of the minor factions in Early. To make the Aragonese (for example) playable, do the following:

Go into your Medieval Total War/Campmap/Startpos directory and open the text file called EARLY. Scroll down until you reach the following line:

SetActiveFaction:: FN_ARAGONESE FT_MINOR

Change it to:

SetActiveFaction:: FN_ARAGONESE FT_MAJOR

You can turn the other minor factions into major factions in this way as well. Next time you start a new Early campaign, the Aragonese will be a playable faction. Make sure you select conquest as your victory condition as there are no glorious acheivements set for the Aragonese or the other minor factions.

If you buy the Viking Invasion addon, Aragon, Hungary and Sicily are playable factions from the beginning.