Usually nicknames are only inevitable when the given name is long, for instance: Josie for Josephine or Alex for Alexander. He may be called Luke but I wouldn't expect it to be a daily occurrence. Lucas isn't exactly difficult to say, is it?
Aldous is fine by itself.
I don't like Marlon (Emmerdale?) but I like Samuel.
I would choose: Aldous if you're feeling adventurous Lucas if you're feeling on trend Samuel if you want a classic Marlon if you want something a bit wacky
thanks v much for your feedback, everyone. It's true I'm probably overthinking the name shortening thing - you can't plan nicknames, can you! If I get a chance, I'll update you in a few days as to what we actually choose (probably be a girl, after all this faffing about)...