Of your options, I like Marshall the best. Marshall Allan would sound great. Stanley and Arnold are not bad FN options either. Owen is very popular right now, and while I love Graham, I don't think it's as timeless as some of your family names.
I love Stanley, Allan, Marshall and Arnold and think they all pair nicely with Frederick. Owen is overdone and Graham is nice but I don't love it with Frederick.
Stanley Graham and Marshall Allan are my fav combos
Allan or Owen. Stanley, Marshall, and Arnold are very "old school" names and I would think of an old man when I hear it...not a sweet little boy. Graham is a good name, but I associate it with cracker, personally.
I love your family names so much more than Graham and Owen! I love Stanley, Arnold and Alan. I also love your first son's name. If I didn't already have an Eddie (Edward) I would like Frederick for our next one!
I'm also pleasantly surprised by the positive response to Stanley and Arnold. I wasn't s huge fan of Frederick at first, but now that he's a real person, it just fits him so well.
Re: Sibling name for Frederick David
I also like Marshall the best, because it sounds more southern to me (and I'm originally from the south)
Stanley and Arnold are both old school, but feel REALLY old school to me, so I'm iffy on those
Stanley Graham and Marshall Allan are my fav combos
Baby #2 Due 3/7/20
Douglas Allen
Vincent Allen
George Stanley
Bradley Arnold
Thomas Marshall
Leonard( nn Leo) Allen
Ooh I really like George and Bradley!
I'm also pleasantly surprised by the positive response to Stanley and Arnold. I wasn't s huge fan of Frederick at first, but now that he's a real person, it just fits him so well.