If you look at the prompt while using a mirrored polygon drawing tool, you'll notice that it offers you to hold down the control key to create smooth nodes. This allow you to create a plan that has both curves and straights in the same poly.

As Wyvern says, you can add pretty much what you want. The idea behind this project is that everyone can bring their own ideas to the table. A local map should obviously be consistent with the regional map it is taken from, but the general idea is to make it interesting by adding features and locations that wouldn't have been mapped at the greater zoom of the parent map. The marked city locations on the parent map are naturally going to be the largest cities in the region, but as you drill down to more close-up maps, smaller cities and other interesting locations will naturally start to appear. So make it yours, add your own lore to the area (if you want to), and make it an interesting place for subsequent mappers to pick for their maps.

Maybe not exactly the way you describe, but you can add large amount of text to your map using map notes. These don't display in-map, but in a separate window, but you can make hotspots to click to show the note you want, and each map note can have up to 8000 characters. Not quite unlimited, but room for quite a bit of text.

I do this frequently for atlas maps, for example, try downloading and opening the Southern Scar map from the atlas, and hit the Description link in the sidebar, and you'll see a description of the map appear. This is a standard hotspot that can be placed anywhere in a map.

Some more information in this blog article:

I used them both in my "Visiting Mars" and my "Starbase Odin" videos.

Unfortunately, while "vector artwork" is a generic term, it is generally not compatible across different programs. The name vector really just means that the artwork is represented as mathematical coordinates so that it can easily be calculated at any size and transformation, but the similarities end there. Every program have their own features that they can apply to the vector artwork, and those features naturally only work in the source program. So even if you could import vector artwork from inkscape, you would still lose all the features of the artwork that inkscape supports that CC3+ doesn't Generally, this means that making vector artwork in an external program isn't really going to do anything for you, just make it in CC3+ directly instead. If you can't do what you want in CC3+, then that feature wouldn't have worked when importing the artwork anyway. When going cross-application the raster formats (png) are generally your best option, while they lack the scaleability vector atwork has, they look the same everywhere because their cintent isn't computer locally.

As for the FSC format, that is the fastcad file format, which is what CC3+ builds on. I don't think any program but fastcad uses this format, and there is no advantage to making them in fastcad over doing it directly in CC3+, the tools are basically the same anyway.

CC3+ can import (older) autocad files, which many programs support exporting to. This is the main option for importing vector artwork, but all the caveats from above still applies, most of the features that another program like inkscape can offer won't survive the transfer.

CC3+ also have some work in progress support fro importing SVG files, which is a popular vector format used by a lot of other software, but again, it is subject to the same restrictions as I discussed above.