I think they sound fine together.
I think you'd kind of be setting yourself up for a pattern, but that's not necessarily a bad thing, just something to keep in mind if you want more children...
Personally I wouldn't use names that had so many similar sounds (the strong M sound, and ending in -lia), but if I saw it on someone else's kids I wouldn't think it was weird or anything. I think Magnolia is a beautiful name, too.
I think they are fine together, and love both names!
I think they go great together! And I love the nicknames. Awesome choices!!