Would doing crossfit and be good for someone with rheumatoid arthritis?

Yes and no: Cross fit is a very intensive form of exercise. If you are not having a flare up of rheumatoid it could be a good way to get exercise. Just be careful to not use too heavy weight and stop when your body tells you to!

Crossfit scaleable: Anyone can participate in crossfit exercises. Workouts can be intense, but can be scaled to your abilities. Acertified crossfit trainer at an affiliate gym can help modify workouts to suit your needs/injuries. Strengthening your body can be helpful with ra. Make sure you allow for plenty of rest and recovery, however and emphasize form and technique over weight and speed during wods.

Possibly: I haven't seen any particular studies of Crossfit and RA. But there is plenty of data that exercise improves pain control, moods, bone density and longevity in RA patients. Be aware, that depending on the severity of your RA, you may require significant modifications of any Crossfit workouts to allow you to participate.