There are "rules" but they aren't really rules. They are a collection of questions that need to be answered by the pot and tree combination, involving style of tree, overall image you're trying for, what you're willing to pay for the container
(cheaper containers offer no where near the visual effect that more expensive pots can provide), ect. The biggest thing to consider is will the rootmass fit inthe pot you're considering?
Colors, shapes and visual weight should all factor into which pot to use. This is an inexact and largely intuitive thing. Go with what "looks good."