The smaller viola case will probably work. You can use foam at the ends to fill the gap. I have a viola case that is designed that way, to hold different size violas. It is not a cheap case, and the foam is not wimpy. You just use the foam to adjust for the shorter lengths of the smaller violas. I upgraded my viola earlier this year and also went from a 16” viola to a 15 1/2” viola. I am using the same beautiful case, i just have the foam insert in use now. It is at the bout end and thinner strips on each side. I don't think there is any at the pegbox end. It didn’t come with foam to secure a viola smaller than. 15” from what it lools,like, but if you need one smaller, just check out a smaller viola case. A case like this is probably made to accommodate the 15” and smaller violas.

You have to make sure, the body length, excluding neck, of the violin will fit the body, excluding neck, length allowed in the viola case. I don’t know the length of a violin back off hand, compared to a viola. Just do the measurements.

If you are getting the case online, ask them before ordering them. Tell them what you have and they can match you up. I would highly recommend the Fiddlershop because I know from personal experience they are extremely good with customer service.