As far as I know, you can't delete the default units. Personally, I edit the default units before I start making new custom units. Wether it saves space or not though, I dunno.
Un-edited default units do not add to map size.
They aren't part of the map since everyone already has them.
The loading time on an edited Footman is just the same as on a custom Footman.
If the unit appears on the map, it needs to be loaded.
If that unit never shows up, it will not.
The difference in size between a default edited Footman and that same Footman as custom unit is the new unit ID (like h001...).
Yes, default things dont add on the size, they are gathered inside the MPQ files, it means your map does not include them actually, they are like shortcuts only.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.