I love your list! We have very similar taste in boys names!
Graham -- Very sophisticated and handsome. I see a Graham being quite popular and smart.
Alec -- This name has a really cool sound, and I like it with Marlo!
Conor -- This one isn't my style for some reason, but it is nice with Marlo!
Holden -- See above
Reid -- LOVE this one! So handsome and straightforward.
Ford -- This one has a very similar feel to Reid but because of the car company I much prefer Reid.
Nolan -- Great name, but I think it has a little too much "o" emphasis to fit well with Marlo.
I hope that helps!
Graham - I see this as a really dated name, sorry.
Alec - This sounds lovely with Marlo.
Conor - I like this with Marlo but prefer the double n spelling.
Holden - Not my style at all.
Reid - My favourite of your list and I think Marlo and Reid make a really cute sibset.
Ford - Not keen on this, sorry.
Nolan - This makes me think of the director Christopher Nolan, so I see it as a surname.