I think soy milk can cause gas. I would think rice milk would be okay since rice is a staple safe food around here. I don't think it will help your constipation or gas, but it shouldn't make it worse.
I don't know if I have heard of anyone using linseed for gas, but back in the recesses of my mind, I want to say that I saw where linseed oil was a bit of a natural laxative. Google it and see if you find any connection. If it is indeed a laxative, it would probably help your gas problem by getting stools (and gas) out of your system more regularly. Aloe vera is a natural laxative, I know, but you should treat it like a chemical laxative, which is to be careful that you don't get dependant on it.