Or, you could feed an "all life stages" food for all the
cats.

That is definitely NOT good advice. All life stage foods are formulated for
kittens and may contain nutrient levels (of fat, phosphorus, calcium,
magnesium, sodium, etc.) not appropriate for senior and/or adult cats.

Canned is definitely better for them than dry food which tends to be
more cereal than meat.


Canned is a good idea but not because of the "cereal vs meat" argument. (what
kind of cereal is that? Frosted Flakes?? lol) Canned foods are mostly water
and getting enough water into the cats should be a goal of proper nutrition.