Just adding the buttons at positions 5 and 6 should work without problems, because they are unoccupied and in the InitialVisible range. Else I don't see much room, because 32 is the maximal possible number of buttons.
I would make the buttons in an increasing order. For sets without InitialVisible the buttons are communtative, but it might be not the case for sets with InitialVisible.
To remove the capturable objects, you have to edit the maps, or just use original maps from the base game, where there aren't the new capturable objects. To get your fortress as a starting building, you have to reference it in the playertemplate "StartingBuilding = ". This object should be one that unpacks a base in a similar way that castles are unpacked.