These are my ideas about spirit guides.
Spirit Guides are souls that guide humans through their life on earth.
What are Spirit Guides?
Spirit Guides are all human spirits that choice to guide someone instead of being reborn. Most of the time the guide is someone close to the person from a past life such as a family member or lover.
They also can take the shape of anything, such that it is comfortable with the person they are guiding. As well as change their name since it is just that a name, so that they can appear to be anything to their person wants them to be. For example, a person might have a spirit guide from the Middle East or Asia but is reborn and raised in a way that they are more connectable with an Indian spirit. Thus their spirit guides changes their name and appearance to satisfy the person. This also means a person could appear to have multiple spirit guides when in fact they only have one taking the appearance of several different people. Basically a spirit guide takes a physical representation of a person's desires, beliefs and ideals.
As for angels and demons guiding people, I really doubt this. You see you must ask the question, Do you really think they know how someone should live their life if they never lived before? As well as if they are such strong and important beings why would they spend all their time with one person, they should have things to do. Thus these "angel" and "demon" guides are all just human spirits with the desired physical representation of the person they are guiding.