You may want to look at 3leggedthing.com - they have a few models that offer a detachable leg. If memory serves, they are stocked by B&H.
On your choice of lenses - my experience is that you need a very robust tripod for a 400mm but can get away with a reasonable monopod. (For the physics inclined, a long lens / camera combination has a large moment of inertia. This interacts with the torsional stiffness of the tripod to lower the resonant frequency.... and low frequencies tend to have large amplitudes.) A monopod is much stiffer in torsion, so it doesn't feel the pain.
I have a 3leggedthing Adrian, and I have been happy with it so far, though I don't have any largish lenses. I have't used the monopod feature, but it's nice knowing it's there. They have a lot of options depending on your needs/budget. I got mine on closeout from Adorama (older model).