OK, I'll try to answer what I can. The spreadsheet (I've uploaded it again below) lists what terrain is unsettleable by the lines for City (yes/no) and colony. Cities cannot be built in Desert, Tundra, Mountain, Forest, Swamp. Colonies can be built in all but Swamp and Marsh. Most units ARE considered wheeled in this mod, so only a select few units can enter Mountains, Swamp, or Marsh. Those that can are basically the archer-type line of units and workers, (and barbs), they are all denoted on the TAM-Stats page with the letter M in the abilities column. A few UU's also have the ability, like Pelatasts, Dacians, may be others.
Note: Swamps/Marsh cannot be cleared until a late tech (Architecture - Second Era), so it will be a while before you can access those. Also, note that Cities CAN be built in Marsh, but settlers can't enter, so you would have to have a worker enter, build a road, then move the settler in to settle. (Awkward, but it simulates the difficulties in settling those areas).
As for your Desert/Agri mention, not only can you not settle them, but Desert can't be irrigated either. (It can be mined or roaded). This was intentional, as the AI otherwise floods the desert, and is offset by having multiple Oasis and plains tiles along the edge of the desert (a mined Oasis is a pretty good tile).
Content citizens should be the same as normal Demigod. Be aware that this map has less than 8 luxuries available, IIRC, and they are scattered, so it's unlikely for any one civ to access more than 3-4. Also, there are no improvements that multiply the luxuries effect (like the standard market), except for the Nubians who aren't in this game, and I think there may be a wonder with that effect. So happiness in general is a little scarcer, but city sizes are smaller as well.
If you don't already know, city sizes are:
Town: 1-4, City:5-8, Metro:9+. Aquaducts come with Architecture (early 2nd age), Metros with Bath House (Medicine), 3rd Age. So founding on fresh water is even more vital, building settlers that take 3 citizens out of towns capped at size 4 is tough!! (But it does limit the AI's REX tendencies as well).
As for Stapel's comment on city flips, the main cause of that is the lack of early culture. Other than the Palace, most civs will have little or no early culture until the end of the 1st Age (Temples), and they are difficult and expensive (require cult site first). Meanwhile, Wonders (AI-specific and Great) generate a TON of culture (most are 10cpt, and at 50yrs/turn double quickly), so if one civ builds an early wonder or 2, they can really rack up cultural pressure quickly. Again, that was an intentional decision of Thamis, he wanted to emphasize the importance of early wonders.
Any other questions, let me know, I will be away tomorrow during the day, but will be back by evening. I hope everyone has fun!