I have a room that's a bit larger than yours and run the Alto Utopias. I've also run them in smaller rooms than yours. Obviously they do better in larger rooms though not too large. I think that given the modest power of your amp, and assuming you can get your speakers away from the walls, add in some room treatments, especially some absorption above and in the corners and diffusion behind, the Scalas should do fine in your space. Trying to dial in the subs with the Diablos is going to be a difficult proposition. I'd use the sub for watching films, assuming you do so with your system. Meanwhile, trade up for the Scalas if you have the opportunity. You'll be happy you did.
By the way, I would strongly recommend if you are not doing so to use the Cardas method for setting up your speakers. That system will help you align them properly in relation to your walls. You can get the details at the Cardas website.
By the way, I would strongly recommend if you are not doing so to use the Cardas method for setting up your speakers. That system will help you align them properly in relation to your walls. You can get the details at the Cardas website.