The ones that tend to cause gas are cabbage and broccoli. Foo had an issue with collard greens, they caused gas with her. I don't know about out rabbits.
The stuff that is high in calcium is, spinach, carrot tops, dandelion greens, kale. Since Thumper is an adult, you don't want to feed him too much calcium rich stuff anyway. Adults don't need as much calcium.
You also have to watch out for stuff that high in oxalic acid. Things high in OA are, kale, spinach, mustard greens, raddish tops, parsley, swiss chard. Its a contributor to bladder sludge.
So the suggested way to feed veggies/herbs is by feeding 2 that are low in calcium/OA and one thats possibly higher in calcium/OA. For example you could do, spinach(high in OA/cal.) paired with cilantro and green leaf lettuce. Or you could do, parsley(higher in OA's) with red leaf lettuce, wheat grass.
See what I'm saying?
Its also suggested that you add a green into the daily mix that is higher in Vit A. Like, watercress, collard greens, endive, mustard greens. A lot of the things that are higher in OA/cal are also higher in Vit A. Like spinach, kale, carrot tops, parsley.
BUT with all that being said, the amounts of calcium and OA are relative. You have to take into consideration the water content in each of the things you're feeding, that dilutes the amounts of OA and cal that they're getting. Also, the amounts of anything high in cal or OA would probably have to be pretty high to do any damage.
When I had Foo on a no pellet diet, she ate a lot of different things. I tried to add in something that was high in calcium, higher in vit A, then other things that were lower in all of those things. I tried to alternate things as much as possible too.
Hope that helps! If I'm wrong about anything, you guys feel free to tell me and correct things!