I think a lot of the answers to your questions are simply a matter of preference. Game modeling has some strict rules, but with high poly modeling to the best of my understanding you have a bit more freedom. That depends on how you want it to look. Are you okay with having identical columns? Are there any asymmetrical…
Cyrael, circular biuldings can be made of modular pieces the same way as rectangular buildings. Like you said, it's an 8-sided repeating pattern. You can then dress it up with decals, vines, etc. to disguise the pattern. Kevin Johnstone had a post about how he made rounded modular bldg pieces, more in this section.…