Ok, so my friend asked me today if he can use for example King of the Swamp with Neos to trigger "contact fusion" (somehow that fusion technique name is one of few things I liked in GX) and summon one of Neos fusions. I looked through the rules and found nothing saying that he can't do that but we both though "so why isn't anyone using that?" So here's the question: is it possible to use substitute a monster for a Neos fusion?

No, you cannot use Fusion Substitutes for Fusion Monsters suchs as the VWXYZ fusions, Gladiator Beast fusions, and Neos fusions, because those cards aren't Fusion Summoned. Because of that, they do not give the used monsters "Fusion Material" status, which all the Fusion Subtitute monsters require.

I hope that makes sense, because I felt like I explained it weird... But in any case, you can't use Fusion Subs for those kinds of summons.