Batteries tied into a full-time battery bank have to be as close to identical as possible. Same brand, model, size, age - hell, even the same manufacturing batch and date if you can swing it. If they aren't, one will do more work than the others and get worn out sooner. Then the next strongest will do the same. Rinse and repeat for a few years and you end up with a prematurely worn out battery bank. They live and die together.
For batteries tied only during charging, it doesn't matter. All that matters is that they require approximately the same charge voltage and that the charge source puts out approximately that same voltage.
Lead-acid batteries aren't precise electronic components. They are sloppy, goofy chemistry experiments in a plastic box. Most of the time, "close enough" is good enough.
Which is exactly what most voltage-regulated alternators do anyway.