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| p.1 #2 · Background Lighting Question |
"Sufficient power" depends on many factors, including the ISO setting you use, the aperture you shoot at, the power of your key and fill lights, etc.
The compact A/C slave strobes usually have built-in reflectors, so flagging to avoid spill isn't required. Most have fairly narrow spreads, but there are some that are 180-degrees or more.
As for tirggering, if you use Speedlites in Manual power mode (no preflash), then simple optically-triggered A/C strobes can be tripped in sync with the Speedlites.
Here's one light that would work in the extension device you show:
Note that these kinds of screw-in strobes are almost always single-power units, so you need to measure the background level and then adjust your other lights accordingly.
Frankly, I can see a number of problems with your idea, including relative difficulty in aiming and placing the lights, power level, etc., but if you want to try it the options are out there, and most aren't terribly expensive.