Guidelines
Some thoughts on what will work.
Models
Each unit must have some models for it, preferably with pictures. They might be old models, existing ones, or conversions. If conversions, it is helpful to give details of how to make them.
Balance
Part of the problem of a ‘mercenaries’ codex is that it can be used to fill in gaps in a player’s army. This can’t be entirely avoided, but paying attention to a unit’s background, and trying to avoid duplicating existing units from other codices can help. No unit should be so powerful that it becomes an ‘obvious’ choice.
Each unit must be standalone (i.e. there will be no armoury or wargear list), and should not have many choices. They are supposed to represent specific units, not generic forces that can be tailored.
As in the Warhammer Dogs of War, each unit will be lead by a character, who gives it its identity. They should be included in the base points cost, and be roughly equivalent to a veteran sergeant or Exarch.
Background
The galaxy is a large place, so you can probably work out how to fit the unit into the background. However, Warhammer 40,000 does have a lot of established conventions, so don’t break them.