Are there guide lines to custom species? Or by making their own is it just like a variation on existing ones? And what kinds if professions can people's' characters have?

There will be guides for sure.  There are “rules” on what species exist, and how their pieces are put together, but the rules are so loose that there are near infinite possibilities.  Basically, the guide will help to explain things to keep in mind as you create; in doing so, you’ll make something that could fit in.

Professions are also basically limitless.