How viable are my Julia and Ninian?

Obviously you didn't pull optimal IV's, but both units are really good to have around, especially with the new arena assault being a thing. You basically need 7 Reinhardt counters for it, and Julia happens to be the premier Reinhardt counter.

Same goes for Ninian, she is a dancer and thus very valuable, her IV's do not matter THAT much because she isn't going to see combat in most cases. While it's tempting to utilise fortify dragons, I'd definitely keep your first Ninian as a dancer for arena assault, and other multi-team modes.

keep Ninian, definitely. Her IVs aren't the best but that doesn't affect what Ninian's primary contribution to a team, which is dancing. So I'd say keep her, maybe until you by chance summon another Ninian? I just feel like she's definitely worth having.
Who else were you planning to put on your dragon emblem team? Would Ninian not fit it well? You only listed three units so I was just wondering, because if you add Ninian it would give you fortify dragons AND a dancer, which would be pretty handy.
And regarding Julia, I use her on one of my main teams and she's pretty good! She's pretty good with Fury and G Tomebreaker. Her tome alone makes her very useful honestly. I highly suggest utilizing her, and she should be fine with neutral IVs.