Not 100% sure how you do scaling, but I have had captains ask viewers to place flagbearers to scale down, and other captains ask viewers to only place level 1 units, to prevent scale ups. If unit type and unit level effect scaling, I think that is counter to the game's design. As the game has many units, why limit players to just flagbearers. The grind of the game is to level your units, so you can make duplicates and souls, why leave them at level 1?
My suggestion is that all units have the same scaling level, regardless of level or scroll type. The more total units are placed, the more the battle is upscaled. That way, if a troll tries to upscale you by placing balloon busters, all it does is puts that legendary unit on a long cooldown for the troll. As far as the captain is concerned it is as if the troll placed a flagbearer. Scaling in this way would increase the skill in the game, as a captain can request specific units that are needed to win, and once those units are placed, they can start the battle early, without worrying about upscaling. Captains that want to play stream raiders as a side game, are not penalized as they would have their viewers placing more units, causing more / higher level enemies. This would put the ball in the court of their viewers to place the units needed to win.
Implementing my suggestion, captains still have control with downscaling, as instead of requesting nothing but flagbearers, they can mark the whole map with "do not place" markers for one battle, to downscale, or they can limit one unit per viewer.
I see the issue of small streamers who only have new players being hurt by this suggestion. So there may need to be a cut off for how many enemies are spawned for very low placements.