I don't see any problem with mixing oils. But when people refer to "essential oil mixes" they usually include essential oils in the mix. Those are different from the oils you mention. The oils you mention are known a carriers and at least one has to be used when using essential oils as the EOs are very concentrated. You can read about essential oils here.
When using essential oils, it's always best to follow a recipe as not diluting them well could lead to discomfort. There are also some considered toxic so it's good to do some research on them.
Nonie is correct--essential oils are very different. Rosemary oil is good for growth (I've read) as well as peppermint; they act as scalp stimulants---these are essential oils. I also read (I do alot of research) to use 2-3 drops of essentials to every 2-3 teaspoons of carrier oils. But since you are just using carrier oils (the ones you mentioned) your mix should be 50% jojoba since it most mimicks the body's production of sebum. Check out Motowngirl.com (i think). She breaks down the benefits of the more popular carrier/essential oils.