Your food cap limit is in Data Editor -> Races -> Terran | Protoss | Zerg -> Food Ceiling.
Nexus -> Movement: Pathing Footprint.
If you change this to 5x5 (Doodad) you will be able to place the Nexus next to the minerals. If you instead want to decrease the amount (or increase it), you'll have to duplicate the 5x5 (Contour) footprint and edit to your wishes.
Ok I tried doing that and it works perfectly for the Command Center and the Nexus but the Hatchery is a 6x5 model and it needs to be a ''Creep source'' Otherwise you cannot build a hatchery on creep. Is there a way to manually make a ''6x5 doodad+creep source footprint''?Sure, why not.
The 5x5 Contour footprint has an invalid Near Resources flag in the Set One that prevents it from being placed near minerals.