True, a liter of Xtol does 12 films. I've never used Rodinal so can't say... except that obviously developer is less critical in all dimensions if one's looking for an image. And that experimentation is a good thing, essential even, if one were to investigate and learn. The cook books are guides, and ensure better repeatability (and reliability) of good quality.
It's like using photoshop - you can do it by the book and get good results, or you can go wild, throw out the book, and learn. Of course with photoshop you're dealing with pixels only and always have the original to come back to, while with developer a screw up resulting in a non-usable negative there is nothing to back you up.
I can report the following things DO NOT WORK: 1) photo-flo is not developer, you will get very clean and clear negatives
2) 120 film requires more developer than 35mm - if you forget what you have in the tank, you end up with film 2/3 developed, with an interesting border on top.