In the first example sentence, could i say どうぶつ without the TACHI? I mean, many times i have seen words like this or KODOMO without the TACHI, though they meant a plural form (animals, children). So could i omite the TACHI form? Same with the third example りょうしんたち
It might be easier to think of たち less as a plural marker, and more of a marker to suggest the actions/descriptions/etc of a group. As you noted, this is not required when talking about more than 1 of something.
Hopefully, someone else will have a better explanation than this!