I've combined all the starting tiles once in a mega-carcassonne.
This include River 1, 2 and 3 (and the spring tile from the GQ expansion), Wheel of Fortune, City of Carcassonne, Windroses 1 (i think we did use the Windroses 2 tiles, but not the starter), school and of course the standard starting tile.
We only used one of each (apart from the 4 different river springs), to not fill the table with starting tiles only
http://www.carcassonnecentral.com/community/index.php?topic=3219.0It worked very well i think, as it gave a lot of options for the rivers (many river tiles and many rivers to work on), and made for high scoring during our early turns.
I would say combining them can work, but would still keep it limited to 1 of each (apart from rivers). More schools.WoF's, cities, Windroses might not work well.