It would be helpful to know whether your sons use their full names or nicknames most of the time. Since you don't mention nicknames, I'll assume not. Maximillian and Felix are familiar, yet not common. They've got an ancient, Roman vibe, but also a modern international one. They are a bit above the garden variety name.
Harold and Edward--love both names, but, against Maximillian and Felix, they just sound too English/Norse.
Jude--one of my current faves, and I like it with your other boys' names. My only concern is that it is much more down to earth sounding than the his more regal brothers.
Samuel and Abel....also both classics that I like, but with the Latin vibe of your other boys, I think this brother would be odd man out.
Gus--too short against Maximillian and Felix, but I really like August/Augustus usuing the nickname Gus
Asher, Spencer and Darcy....sound too modern/trendy with the other boys
So, my choice for a boy would be August (Gus) or Jude.
For a girl, somehow I don't think the Latin thing is as important, but I would want something a little vintage and with a lot of dignity. With that in mind....
Florence--exactly right, I think!
Eleanor--I like, and I don't think it would be a bad choice at all. But Maximillian and Felix are a little quirky to the modern ear (I think it's the x's), and Eleanor doesn't have that.
Clementine--I'm still on the fence about. I like it, it's got that little bit of quirk, but I'm not sure.
Iris--I like this a lot. It's an ancient name, the starting 'I' makes it a little unusual, it's got that vintage flower feel, and yet it's not a insubstantial flower name.
Anthea (nn Annie)--a little less familiar than her brothers, but I like it.
Beatrice--another winner. Latin pedigree. Dignity. Beatrix would unite the siblings with the 'x', which might or might not be a good idea.
Stella--love it, but, to my ear, Stella is much more down to earth than her regal brothers.
Hazel, Heidi, and Mietta--don't seem to work with this sibset. There's just some sort of cultural/stylistic disconnect to my ear.
Lyra--I like this, and even though it's an old name, it's got a modern feel (maybe because of the popularity of the Lila-like names?) that I don't like as a part of your sibset.
So, my choices from your girl list are: Iris, Beatrice, Florence and Anthea, probably in that order.
My favourites for boys are Samuel, Edward & Abel, but I have a soft spot for Harold too, especially if his nickname will be Harry!
For girls I like all of your names but my favourites are probably Florence, Clementine and Beatrice, especially when nicknamed Bea or Beattie.
I think Harold and Beatrice would suit your sibset the best. Your sons names are very handsome by the way!
I think these names work best with Maximilian and Felix.
Jude (my fave and the best name with your boys)
More predictable but they work...
I didn't choose Gus because it lacks substance as a full name next to your two boys. August nn Gus would be better.
I didn't choose Spencer or Darcy because they're too modern with Maximilian and Felix.
I didn't choose Abel because it shares an "el" with Felix.
Anthea (nn Annie)
I didn't choose Hazel and Stella because they share an "el" with Felix.
I didn't choose Florence because it shares an initial with Felix.
I didn't choose Iris because the s and x endings of Iris and Felix are too matchy.
I didn't choose Beatrice because it's too similar in sound to Felix.
I didn't choose Lyra because it has a trendy, modern vibe.