For what it's worth, I own the MLD Monk Tarp (spinnaker model) and my 6' frame fits underneath it with some wiggle room to spare at the head and feet end. I've used it with a bivy and without in mild weather conditions, mostly to cut wind on late spring/summer trips. I pitch it fairly low to the ground, staked all the way to the ground or raised up 3-6" inches to increase room and ventilation underneath the tarp. I pack the Monk when weather conditions are fair, knowing the limitations of the coverage it provides.
I think you'll find the MLD Grace Solo pitches faster, easier, and more taut, aided by the fact that it has a catenary curve along the ridgeline which gives it a nice strong pitch. However, the Grace is sort of a one-trick pony, there's only a few different configurations you can pitch, mostly adjusting the height of the pitch or raising one side up, whereas a 6x9 or 8x10 flat tarp gives you a wide variety of pitches to play around with and adapt to different conditions and needs.
Your necessity for a bivy is going to depend on your weather conditions, only you can really judge beforehand whether a bivy is necessary or not. A bivy is a pretty standard supplement to a lightweight tarp.