Filtering takes many shapes and forms as there are an unlimited number of variables, so it gets down to specifics. I think of filtering as a series of operations and then look at each of those to see if the filter does that operation to meat what is needed for each tank as there are many ways to adjust the results.
Does the filter remove the correct amount of debris from the tank to hide it out of sight until I get the time to clean the filter. Two cans should, even though they are on the small end but that can be bumped up if we need by adding simple, cheap, powerheads in the places where the debris needs a "kick" to get it moved on down the line. I use lots of the super small powerheads off the auction for $7 as cheap and simple. two filter should give adequate time between cleanings but that depends on your personal schedule and wishes.
How much waste from fish stock and plant material needs to be converted from ammonia to nitrate? Varies a lot from tank to tank, so no firm way to say. I would guess that two small cans could do the job.
Water movement to get CO2, O2, ferts moved to all parts of the tank and keep the temp uniform?
I go with powerheads to do the simple stuff.
I would start with what I have and adjust if the guess is wrong, not change due to some magic number proposed a sure cure for all tanks.