I agree with some of the above comments and don’t find your styles polar opposites or anything. He seems to like more traditionally classic and you like more unusual classic, but I think there’s a way to meet in the middle. Since the girls all have longer and fairly feminine names, I would stay in that style for a girl and not do the shorter ones (like [name]Luna[/name], [name]Stella[/name], [name]Cleo[/name], etc.).
With [name]Alexander[/name] ([name]Alex[/name]), [name]Callum[/name] ([name]Cal[/name]), [name]Penelope[/name], [name]Easton[/name], and [name]Magnolia[/name], I like [name]Harrison[/name], [name]Lillian[/name], [name]Dashiell[/name] ([name]Dash[/name]), [name]Tobias[/name] ([name]Toby[/name]), [name]Desmond[/name] ([name]Des[/name]), [name]Vivian[/name], and [name]Delphine[/name] the best.
Some comments on the names on your list and further suggestions:
[name]Everett[/name] - works fine with sibset, but I would not use because of same initial as [name]Easton[/name]. Alternatives: [name]Elliot[/name], [name]Kenneth[/name], [name]Declan[/name], [name]Malcolm[/name], [name]Russell[/name], [name]Howell[/name], [name]Beckett[/name], [name]Arnett[/name]
[name]Benjamin[/name] - works fine with sibset, but fairly common. Alternatives: [name]Bennett[/name], [name]Gabriel[/name], [name]Gideon[/name], [name]Samuel[/name], [name]David[/name], [name]Brennan[/name]/[name]Brendan[/name], [name]Theodore[/name]
[name]Charles[/name] - works fine with sibset, but I would not use because of [name]Callum[/name]. Alternatives: [name]Frederick[/name], [name]George[/name], [name]Henry[/name], [name]Marcus[/name], [name]Gilbert[/name]
[name]Noah[/name] - It seems a little short compared to the other names. Alternatives: [name]Nathaniel[/name], [name]Philip[/name], [name]Ronan[/name], [name]Dominic[/name], [name]Nicholas[/name], [name]Roland[/name]/[name]Rowland[/name]
[name]Harrison[/name] - works well with the sibset. Alternatives: [name]Henry[/name], [name]Truman[/name], [name]Davis[/name], [name]Howell[/name], [name]Gordon[/name], [name]Grant[/name], [name]Julian[/name], [name]Lucas[/name]
[name]Maya[/name] - I wouldn’t use because of it’s short and has the same beginning as [name]Magnolia[/name]. Alternatives: [name]Layla[/name], [name]Lenora[/name], [name]Nadia[/name], [name]Ramona[/name], [name]Fiona[/name], [name]Isla[/name]
[name]Lillian[/name] - I like this a lot with your sibset! Alternatives: [name]Louisa[/name], [name]Lorraine[/name], [name]Juliet[/name], [name]Juliana[/name], [name]Tabitha[/name]
[name]Lucy[/name] - love the name, but it’s a little short. What about [name]Lucinda[/name] or [name]Lucretia[/name] with [name]Lucy[/name] as a nn?
[name]Katherine[/name] - I like the name, but it’s pretty common; alternatives: [name]Caroline[/name], [name]Cassandra[/name], [name]Cecilia[/name], [name]Cordelia[/name], [name]Clementine[/name], [name]Coralie[/name]/[name]Coraline[/name], [name]Katrina[/name], [name]Corrine[/name]/[name]Corrina[/name]; [name]Constance[/name]
[name]Stella[/name] - Another one that seems short compared to the other girls’ names. Alternatives: [name]Susannah[/name], Serephina, [name]Sabrina[/name], [name]Eleanor[/name], [name]Estelle[/name]/[name]Estella[/name], [name]Helena[/name]
[name]Amory[/name] - love the name, but maybe as a mn? Alternatives: [name]Peregrine[/name] ([name]Perry[/name]), [name]Lawrence[/name], [name]Stanley[/name], [name]Wesley[/name]
[name]Lennox[/name] - [name]Lachlan[/name], [name]Maddox[/name], [name]Lawrence[/name], [name]Truman[/name], [name]Randolph[/name], [name]Brecon[/name], [name]Beckett[/name]
[name]Dashiell[/name] - love the name, especially with nn [name]Dash[/name]
[name]Caspar[/name] - I prefer [name]Jaspar[/name] (and I think that would work better with [name]Callum[/name])
[name]Desmond[/name] - [name]Dexter[/name], [name]Declan[/name], [name]Lachlan[/name], [name]Edmond[/name], [name]Edwin[/name], [name]Edgar[/name], [name]Dillon[/name]
[name]Tobias[/name] - great name; alternatives: [name]Thaddeus[/name], [name]Thomas[/name], [name]Todd[/name], [name]Truman[/name], [name]Malachi[/name], [name]Elias[/name], [name]Hugo[/name]
[name]Ariadne[/name] - while I like the name, I’m not sure that it fits well with the sibset. Alternatives: [name]Amelia[/name], [name]Arabella[/name], [name]Aurelia[/name], [name]Aurora[/name], [name]Araminta[/name], [name]Helena[/name], [name]Phoebe[/name]
[name]Harlow[/name] - this seems like a really different style from the rest of the names so I wouldn’t use except as a mn.
[name]Vivian[/name] - love this with your sibset. Some alternatives: [name]Violet[/name], [name]Victoria[/name]/[name]Victorine[/name], [name]Viveca[/name], [name]Tamsin[/name], [name]Beatrix[/name], [name]Genevieve[/name], [name]Georgina[/name], [name]Roxanne[/name]
[name]Delphine[/name] - Unusual, but I like it with your sibset. Alternatives: [name]Daphne[/name], [name]Dorothea[/name], [name]Theodora[/name], [name]Delilah[/name], [name]Bettina[/name], [name]Rosamund[/name], [name]Rosaline[/name]