Phil, "do not mix" applies, because if you mix them, the old ones pull down the new ones, and you wind
up wasting money as the new ones are just abused by the old ones.
The bluetops and yellowtops are supposed different, with the yellowtops having a thinner plate designed for dual-purpose use (starting plus cycling) while the blues are designed with thicker plates for deep cycling. Which doesn't stop you form using themas starting batteries, as long as they have sufficient power, as that bank would.
But you might want to reconsider using Optimas at all. Usually you can buy flat-plate AGM
batteries for 1/3 less, with more capacity because there are "square" cells using flat plates in the rectangular box, while Optimas are cylindrical cells that waste empty space in the battery
case. Typically a 10% loss, same-size.
Flat plate AGM
batteries are available from many vendors, just ask around. And of course, that's ignoring the entire "Do I want AGM?" question.
But mixing new and old Optimas? That's an expensive way to go. Pointless waste, I'd say.