Yes, you can mix manufacturers for the most part (Murray objects to this practice).
What you can't mix is the Type.
You need to know whose shape is used for the breakers. Removing the panel cover and reading the side of the breaker is a way to find out. The ultimate way - with no other markings - is to remove a breaker and take it to the store with you. But you have to know what you are looking for. There are shape differences that don't matter at all. And there are subtle differences such as between BR and BRD that make all the difference.
