You will be fine mixing brands. Just be sure that whatever fluid you choose, it is labeled as LV fluid. Don't use Dex/Merc, Mercon V, or anything else which is not specifically labeled as low viscosity. Also check the label to make sure that the fluid meets or exceeds Mercon LV specifications.
My next ATF exchange will involve Mag1 ATF. There is an LV formula which I got at a reasonable price. Note that it is licensed by Ford.