Important things to note:
AI and ID have setting which displays rich black differently ( usually darker) than plain K. So what you see on the screen is not necessarily what you will get.
In the above there are also two blacks swatches: 'registration' ( which you know as 'rich black') and K (black) bake use to pick one or the other depending on your final output needs.
DO NOT MIX one with the other. That is if you are doing a 100K-20K gradient make sure is that and not 100K-20CYMK.
in a ONE COLOR JOB the 'color' is irrelevant. What you are in fact doing is mapping % ink coverage. so again as long as you dont mix BASE colors ( this includes NOT mixing pure black with rich black by mistake you'll be OK.