Not a good idea to mix different forms of groundcovers in the same planting area.....you will just end up with a big mess. Growth rates are different for different species so one will always tend to take over to the detriment of another, especially if they prefer different growing conditions, as your choices do. Best to just pick your favorite (or you can go with a couple different selections of the same plant if you prefer) and let it take over. And if you DO choose to pick a couple different varieties of the same plant - e.g., different creeping thymes - plant them in separate swatches rather than intermingling them.

I agree that it is almost impossible to keep a balance among various groundcovers, as they vary too much in their vigor, heights and responses to every factor.
I have used Mazus reptans and Ajuga in the same area if I constantly pull out the rampant Ajuga. My rock garden is constantly in a state of flux as one groundcover begins to dominate another.
Huge, uncontrollable swaths of Periwinkle are on the property, and a concentrated digging and rooting it out only makes a temporary difference. But I notice the taller, more refined, and hardly evergreen Epimediums seem to slowly dominate the Periwinkle.