Is there any restriction about those two factions?
Senate and the Latium region are needed. The second faction can be anything.

I wonder about the senate missions. How are they assigned? This comes to me because the missions for the Brutii, for example, are assigned to regions that have been eliminated in the new map.
That is interesting. I have also heard about assumptions of the AI being programmed to always attack certain regions/factions at the start of the campaign. I hope that removing regions doesn't affect the game too much.
In descr_strat of the imperial_campaign the victory conditions are the same as in the provincial campaigns but the option of short campaign does not appear.
Victory conditions are hardcoded. The entries in the beginning of descr_strat.txt are worthless. The short campaign conditions might be hardcoded to be limited to the Imperial campaign. ...oops, I have now read your edit. So when you overwrite the Imperial Campaign, all the factions should be present to use Short Victory Conditions?

why is the sea in the editor shown as being above the land?
Hmm, when I edited the campmap the custom battles were sometimes flawed as all units drowned since the sea level was above ground. Perhaps it has to do with the maximum height in map_heights.tga. Should be worthwhile to look into.