When stowed, the Flippac is essentially like any other camper shell, you can use all of the space up to the roof. If you need to use it for work items that stick up or out past a normal Leer, or similar shell, then it's not for you. You could fashion a lifting device but they are designed to generally stay on. When I had an empty shell, I took it off once or twice for some odd use. Similarly, I've had the Flippac off a couple of times.
Regarding the number of campers in a Flippac, we've camped comfortably with 2,3,4,5, even 6! You tend to sleep in it and live around the rig whereas with a truck camper you can live in it more easily. On a rainy evening we were able to comfortably play cards and games as a family of 4.
So how did we sleep 4,5,6 in our Flippac?
For 4, we sleep two in the bed, one in the hammock, and one on a sleeping platform below the hammock. Or, we lay the seats flat in the truck and the kids have some measure of privacy in the cab as we have a little in the bed.
For 5, our Flippac walls happen to be exactly 72" wide so when we first got the Flippac, we simply laid two 6' folding tables across it, stored gear underneath, and had three little ones on top. We've also had two in the bed, two in the cab, and one on the hammock.
For 6, it's two in the bed, one in the hammock, one on the sleeping platform I built, and two in the cab. With a little more creativity, I could sleep 7 but the truck only holds six people so I'm not sure how the 7th would get to the campsite.
My truck is a full size truck, so not all of these configurations are possible with smaller trucks or trucks whose front seats don't lay flat. My point is you can really pile people comfortably into a Flippac with a little creativity. But if you need to remove your camper for a completely open bed, the pop up easier.