Mila has exotic European flair and Arden is a unisex gem so it would be nice for her sister or brother to have a great blend too. I would eliminate Ophelia and Thea from consideration as they're both too similar in sound to Mila.
Sylvie - a little more exotic than Sylvia
Calla - one of the more exotic flower choices but a little matchy with Mila (two-syllable names with pronounced l's and ending in "a")
Clara - has vintage charm but not very exotic. Again it has the similarity (Mila, Calla and Clara would begin a pattern)
Octavia - unusual Roman gem
Isis - if you want something totally different
Vesper - reminds me of snake (Viper)
Elodie - this is pretty
Lyra - this name is trendy but I think it's still rare enough to use. It does have a similarity to Mila though (see Calla)
The only mn I like on your list is Elowen.
I would elimate Nelson and Claudius (too clunky), Atticus (trendy), Loki (a trickster god and it's too cutesy to take seriously) and Atlas (just odd) as first names.
Octavius/Octavian - I'm not sure which version I prefer. They're both pretty awesome. Combo Suggestions: Octavius Ambrose and Octavian Pascoe.
Aurelius - I really love this name! Aurelius Ivan would be awesome!
Rouvin - I think this looks and sounds wonderful with Mila. It's unexpected but they work together. Rouvin Casimir would be wonderful!
I absolutely LOVE Mila and Vesper as a sibset! Vesper makes me think of the very pretty woman in the new James Bond movies. Vesper Callia would be lovely. I also really love Elowen, but I don't think it flows particularly well with anything from your fn list.
For boys my choice would be Atticus. I think it sounds fantastic with Mila. Though I would probably go with a one syllable middle with it, and you don't really seem to be a fan of short names ;) From your list I like the combo Atticus Pascoe the best.
And I think as long as you don't name your second something like Ophelia Alexandria Theodora Guinevere, the length of the names shouldn't matter. :)
Lol Leahmarie, I agree! Atticus Fox would be fabulous.
Judyariel - I like Sylvia Lotus and Octavian Ambrose from your suggestions. I think I am leaning more towards Sylvie than Sylvia, though.
I also like mischa's idea of a relatively unisex middle for a girl, but I'm struggling to think of many that I like.