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I'm gonna have to say that this is too much of a stretch, and here are some of the reasons why:
Let's go in order of the points brought up - One, using the legs as a reason for the grass starters to be dinosaurs, while an educated reason, is not particularly valid in pokemon. If you haven't noticed, most reptilian pokemon do not touch their torsos to the ground. Second, bulbasaur's evolution tree doesn't really seem to look like any kind of dinosaur, not to mention, the prefix "-saur" does not mean it has to be a dinosaur, just a reptile. Treecko is most definitely based on a gecko, and its evolutions just build off of that (unless dilophosaurus had back boobs). Snivy and family are based on grass snakes (garter snakes, etc.). Chespin looks nothing like the "first mammal." The only ones that seem to look like dinosaurs are chikorita and turtwig's families.

Next, let's talk about the zodiac theory. Cyndaquil is a hedgehog, not a rat. It's german name even translates to FireHedgehog. Fennekin is a fennec fox, and while those are considered a canine, they are a different species than the dog, so this does not work in the zodiac, either. The other three are based on common things found in most real (or in a dragon's case, fantasy-esque) settings.

Finally, let's look at the proposed water pokemon criteria - "No water type starter is based on fully aquatic animals." So, this narrows down the available type of water pokemon to... oh... any animal that ever enters water while living on land, or vice-versa, ever. I'm looking forward to an antelope water starter, in gen 7. Basically, it's too vague for a good theory.
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tl;dr, while a cool idea, these theories hold no real ground.