Typically you don't create units with missions, for exactly the reasons you describe. Something more like the asteroid/orbital bombardments that appear in existing missions (unstable warp?) is more controllable and less fiddly.
If you're absolutely set on making summoned daemons a part of your mission I'd have each player roll a d6 at the start of their turn. If they roll under (not equal to) the turn number they can set up a unit of chosen daemon type anywhere on the battlefield at least 12" away from the closest enemy unit. During each player's turn units produced this way are friendly, and may be used (move, fire, charge, etc) as if they were part of their army. If one of these units begins their turn within 1" of any player's actual units, whether friendly or not, they are immediately engaged in close combat.
I'd keep the daemon unit type small. Maybe brimstone horrors. I guess it also depends on the "flavor" of your chaos table. Depending on how unstable you want the area, either make it a standard minimum unit or make the unit a number of models equal to the turn number (2 on the 2nd round, which is the 1st round they can show up, 4 on the 5th round, etc.)
Some thoughts.
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