For the canned pumpkin you only need a tiny amount so what I do is freeze it in one of those tiny ice cube trays and then pop them out into a ziplock bag and keep them frozen and then I give one to them on top of their food. A can of pumpkin lasts forever if you do that. I don't know how long it is good for if you leave it in the fridge.
I have pro-biotics downstairs I always forget about but they are great. You have to keep it refrigerated though. The brand I have is The Wholistic Pet. They are a really good brand - no fillers or junk in their stuff. I've met the couple who own the company and they are very nice people. More than willing to educate
To avoid constipation with the rice I would only give like 1-2 tbsp on top or mixed in with the regular food. Otherwise yes it can have the opposite effect