Wetlands are a common in all climates. In fact probably the
largest one is part of the taiga in the West Siberian Plain in Russia.
Tundra in Civ is not an area of frozen ground - that is ice. Tundra terrain covers areas of severe winters and short vegatation period, but forest can appear on it, so I guess it represents both tundra (devoid of larger trees) and taiga (coniferous forests in cold regions) biomes.
Permafrost is quite common in such areas, but it is below the ground during summer or even the entire year. In fact it keeps the water just below ground as it can't go deeper (and even adds some from permafrost melting). Thus, it even helps wetlands forming.
Typically, swamps are considered different than peat bogs. But judging from my experience from northern Finland, peat bogs are still quite rich compared to the vegetation around. So, maybe the general term wetland suits the feature better, but the swamp sounds better and it works a bit alike.
As for the jungles, there are many different kinds of them and only a few are a really wet. Most are just very rainy.
Sorry for the long explanation. Such things are part of my real life occupation and I wanted to explain my reasoning clearly...
There is no easy way to limit feature to certain latitudes. And 70 is on standard settings close to ice/tundra border I think.
As for the centaurs, I found the bug. It was in the CIV4CivilizationInfos.xml The right file is below, but I am not 100% if it will not break the save games.
I rearranged the religions from AV to Order and all is working fine. By the way I tracked the bug that made air mana display in three-field order civpedia. It was from unmodded FfH I think.
I added some new things too. I hope I will release next version tomorrow or on Sunday.