Hi Mike,
I tended to stick to white for a soft fill in, silver reflects more light but can be a bit harsh.
The problem you have with anything living is that they have a habit of going the moment you try and stage a photograph, butterflies in particular, so you need some means of anchoring the reflector quickly.
I have seen a someone mount an arm on the tripod just for that reason.
Also for that reason I would think that you would find anything less than 12 inches a pain to set up as it requires less precision than anything smaller and will cast a better light.
Personally I would experiment with a bit of cardboard faced with white printer paper before committing cash, this would allow you to see what works best for you. I suppose a folded Lastolite still reflects light, I have never tried, but I suspect that it isn't worth the hassle.
The other point you raised of using the reflector as a wind break, the 12 inch would be about the minimum to be any good, however from my experience with the big Lastolites, you will probably find that you need a windbreak for the windbreak or at least a very rigid mount for it. Hence the assistant for my big one!
With anything that moves I have variable power macro flash, which is not ideal, but more certain than trying to rig a reflector.
Like a lot of wildlife photography there are few easy solutions which is why you see so many artificial 'tank' setups used for wildlife films that must get their shot. At least the subject matter is still alive even if it is in a very confined space between two sheets of glass!