I have used some of the "peanut" optical slaves to trigger smaller flash units for fill or background lighting. Any optical / IR unit has challenges. For example, I use ONE radio slave to set off one of my heads and use the built in IR slave to set off the rest. Typical problems:
- the radio-fired unit is set to a low power and the IR can't "see" the flash
- the contrast between the flash and the ambient light is too small and the IR won't fire
- the units are set up in such a way that the IR can't see the primary flash and the IR won't fire
- I've had two units across from each other and the MODEL blocks the flash and the IR won't fire
Sometimes I can adjust the ambient light or the position of the strobe head. I have between one and four heads in the shot -- so sometimes I have to pick a different strobe to put the radio slave on. Sometimes it's the hair light because that one is high and behind, and the IR unit on the strobes can "see" it better.
If you want a 100% reliable trigger, you should buy a radio unit for EACH head. Since that costs $100 a head.... I use the IR built into each unit. I'm shooting with Calumet Travelites and have found them pretty reliable. It's almost always the moody figure or fashion work that becomes an issue -- again, with a low amount of light from the unit that is radio-attached. It's easy to fix with a little planning.
Something with a low light and two opposing strobes: