Other Heads on the Viola Body
Viola's headback will hold the faceplates for Unoa 1.5 girls (Sist & Lusis) and I assume the boys L-Bi and B-El too. However, the fit is not perfect, Unoa faceplates are a bit loose on the Viola headback. There is some gapping, but this can be fixed and the faceplate secured by a little bit of poster tack/eye putty in the little grooves on the side of the faceplate.
For newer Unoa releases, the Unoa chibis and elder sisters, the faceplate system is slightly different and will not fit on the Viola headback. If you'd like one of these faceplates on a Viola body, or if you don't like the fit of the other Unoa faces on the Viola headback, I recommend getting a headback from Buff Dolls. This headback fits well on the Viola body (it sits a tad higher up on the neck, but doesn't look weird) and it is sculpted to fit all Unoa faceplates, including chibi and elder sister, snugly.
Here's a video showing the head system and how the Viola body works with Unoa faceplates:
Because I don't own any other dolls besides Unoa, I don't have any definite answer to what other heads will fit. However, since I know the Buff Dolls headback fits on the Viola neck fairly well, I would assume that most MSD size heads would fit on the body just fine.
I also think some larger heads might look cute on the body, like a super deformed chibi character! But since I don't have access to larger heads I haven't tested it out...
Once the Viola preorder starts shipping, I'd love to see some different heads on the body! Leave a link in the comments or send me a message and I'll try to update this article with more info!
Viola Head on Other Bodies
The Viola head should work on any MSD body that has a normal neck system. There might be some gapping, depending on the thickness of the neck. Her jaw is also fairly wide compared to other slim minis, which might make her head look a little large. Here she is on a Unoa body:
For Viola to fit on Fairyland MNF bodies, either the neck system would need to be removed from the body, or the Viola headback would need to be modified to work with the lock & key mechanism.
Viola Next to Other Dolls
I think Viola fits in well with typical "slim mini" type dolls like Fairyland MNF and Unoa. Her features (head, hands, etc.) are probably a little too large to look right next to 1/4 scale fashion dolls or similarly sized BJDs like Iplehouse FID or Dollshe Fashion sized dolls. Here are some pictures of Viola next to other dolls: