Yes, a rabbit may still poop and eat even when having gas. It's when the gas pain causes them to stop eating, that the pooping will also stop.
Dosage is 1-2cc(20mg/ml). 1cc for small buns, 2cc for med to large buns. Every hour for 3 doses, then every 3-8 hours as needed. Generally if a bun isn't feeling well and it's been less than 12 hours since they last ate, then I'll let them run around for a few minutes to see if that helps any and maybe a gentle tummy massage. If not then I'll give simethicone. But if after the 3rd dose, or 2 hours, they aren't feeling better, or it's been more than 12 hours since they last ate, then it is time to take the bun to the vet. Or if there is noticeable bloating, vet immediately.
http://www.bio.miami.edu/hare/ileus.html
You may also want to get a different brand of the simethicone. Something without the artificial sweeteners saccharine, sucralose, xylitol, and aspartame. Sorbitol is ok.
Rabbits almost always require higher med doses per lb, because of their faster metabolism, though I'm not quite sure how it applies to simethicone, as it isn't absorbed into the bloodstream, but stays in the GI. It is considered a relatively safe med though, and even giving a bit too much shouldn't cause any upset. Though with any medication there is always the possibility it can cause an issue.
