You'd be fine just adding other batteries. Make no difference having D series and Xp batteries in your bank unless you want to get OCD about it
Just to clarify the XP series as a starter battery though, I want to say I read they're just not meant for under the hood use because they are not as tolerant of heat as the D series are. The D series are built beefier and are able to be used BOTH under the hood AND in a bank. The XP series is ONLY to be used in a bank, away from the heat of the engine. I'm pretty sure you'd be fine running an XP series battery as your singular main starting battery as well, I had one XP3000 as the only battery in my Cavalier for a while a couple of years ago before I beefed up my bank and it worked just fine. The only difference being it was in the trunk, NOT under the engine.
Tl;dr-D series able to be used under the hood and in the back in a bank; XP series for locations away from heat ONLY(such as trunk, interior of vehicle, NOT under the hood).