You cant mix batteries of different capacities or different ages together. Victron go to a lot of trouble to ensure that the individual cells in the battery get balanced and then they want you to go to a lot of trouble to wire them in such a way that all the batteries get charged equally and do an equal amount of work (by making sure they are wired identically with cables that match in length) I hope that goes some way to explaining why you cant mix different sizes and ages.