The poor battery against the better battery is not just theory, it's a fact. When batteries are hooked in parallel, even though they are charged above their normal voltage, when the engine is turned off, one battery will hold less charge than the other, and they will tend to equalize sitting there over time, ending up not as good as the better battery, and not as bad as the worst battery.
I really suggest you look into battery theory some more.
If I installed 1 group 31(850CCA) and 1 Lawn & garden battery(200CCA), the lawn and garden battery /will not/ discharge my group 31 battery to "equalize".
Remember that a 12V battery is made up of 6 cells, each cell being 2V nominal. Whether a tiny little lawn and garden battery, or a massive semi truck battery, they both are the same chemistry and each cell is 2V... The only difference is the size of the plates in each cell.
The only way you get one battery discharging into the other is if:
A. One is discharged(at which point the battery /voltages/ will equalize, with the dead battery charging off the good one - a small dead battery won't drain as much as a large dead one will).
B. One battery has a shot cell. If one of the cells is shorted(something that ends up happening in a lot of batteries that 'die'), it will have a nominal 10V, peak 11.5V or so.
Because that is lower than the nominal 12V, peak 13.5V from a 'good' 12V battery, that battery will charge the 'dead' one, which can't actually accept the current(as each of it's 5 good cells is already fully charged), and so it ends up electrolyzing the water in the cell until the voltages equalize... around 11.5V.
This is at the 'dead' point of the good battery.
As such, you have to keep a shot battery away from a good one, if the shot one has a dead cell. If they both read 13.0+V after charging and letting sit for an hour, even if one barely has enough capacity left to light a bulb, it won't discharge the good one... the bad one just won't do anything when you go to use it.