I would definitely bring a monopod. Where a tripod might be difficult to use given space constraints, time constraints, or regulations of the exhibit, a monopod should give you a considerable amount of stability while still being very mobile.

If you are inside of the glasshouse (green house), you should be free to use flash. I would either use a macro flash, or find a way to move your flash off camera, close to the subject, with a small diffuser. That should help increase lighting, which even in a green house is likely to be dimmer than you would really need. You won't need a large light...but a small diffused flash should be perfect for butterflies.

If you are going to be shooting through glass, you'll want to make sure you shoot at an angle that hides reflections. It can be possible to use a flash through glass, especially if it is off camera, but you run the risk of increasing glare and obscuring your subject with reflections. You'll want a fast aperture and higher ISO if there is limited light.