Not playing either of them, but in my view, being a DM is all about taking the pieces you want for your own campaign and mashing them together, no need to feel locked into one particular world or setting.

Symbols keep their layer when defined, and the entities should normally be put on the SYMBOL DEFINITION layer before you define them, this won't happen automatically. As for the Common sheet, that doesn't really matter that much, since sheet information is lost when defining the symbols, but having all entities on the same sheet is a good way of seeing that things look right, otherwise the actual entity order in the finished symbol may not be as you percive it since sheet ordering trumps entity ordering. Using Common is a good practice, because Common isn't a sheet at all, it is just all the entities that doesn't actually have a sheet, which is exactly the situation for a symbol definition.

For the catalog, yes. A good way to build a custom catalog wich includes existing symbols is to first make a catalog with your symbols, using save as catalog from the symbol manager. Then, open this new catalog up as a map (as opposed to in the symbol catalog window), and then open up the catalog with the symbols you want to insert in the symbol catalog window, clicking on each symbol once to import it into your own catalog. Then simply save it (using the normal save button this time, no need to to the symbol manager route every time).

If they are pixelated, it is usually because the scale for the fill in the fill style dialog is way to large. Try reducing it for the fill styles in question.

You can also force CC3+ to use a higher resolution version of the file through the Display Speed Settings.

Through the cog in the top right corner of the post. Note that you can only delete a comment, not a topic. The latter is due to how the developers wrote the forum software, since deleting a topic would also mean deleting all the comments of the topics (which probably don't belong to the poster of the topic), so they simply did not allow users to delete topics (even if there are no comments yet)

Actually @jslayton, it is the opposite way around. If you don't hold down shift, all effects are wiped and the new preset is applied, but if you hold down shift, the current effects are kept.