I think you can definitely add a different tile behind the stove, and your reasoning sounds...well, reasonable. Plan it into the design and make it look intentional and attractive and it'll work fine. Maybe run a line of your pencil molding through your marble tiles to connect the two... But that's just one of the ideas to make them connect and coordinate. Your area behind the stove can become sort of a feature or focal point, even if it's subtle.

I don't think it's quite what you have in mind, but I have 3 different backsplashes in my kitchen...2 mosaic designs using the same types of stone, but in different designs and use of the colors, and larger slate/quartzite tiles behind my stove.

Here is a link that might be useful: Pics of my kitchen

Thanks. I thought about trying to tie the two areas together with some sort of mixing, but perhaps a pencil liner in a contrasting color would work well.

Your pictures spawned another idea -- I see you used wood to frame out your range area. I thought about using something like corbels or columns, but I was wary of code violations. I see you offset yours by the required clearance.

Haha... There are actually supposed to be corbels there where you see the wood, but we haven't finished them yet. The joys of DIY homebuilding. ;) But I've gotten used to the wood dividers and kind of like the simplicity.