what's the difference between taimid and ta muid?
Nothing. They're just used in different dialects.
Didn't think this was a dialect thing, just the natural shortening that occurs when speaking. E.g. "it's" versus "it is".
Nah, it's more dialectal. You won't hear, for instance, táimid in Ulster or often in Connemara. But it's likely all you'll hear in Munster (which also uses other synthetic forms).