Thanks for following up. Writing on this subject helps me reflect on my technique, refresh notions, and keep a clear mind. Might help some people as well.
snip: If I set them on auto, what happens as each flash tries to meter its own light, plus whatever light comes from the other flash?
That's what they'l do on auto, for sure. I was suggesting using them for backgroung and effect, such as rim lights, because you may like to use the Advanced Wireless System for main subject lighting, and the old flashes for the other "subject" wich is background. Because those areas are not or only dimlit lit by the front SB-800's subject lights, auto flashes will try to maintain a normal exposure for background by filling in whatever is required to complete the exposure. They should have very little influence on the subject front lighting.
snip: Also, if I want to use the older flash to light up a background, (and let's say the Guide Number is the same for all units, just to keep things simple), is there some sort of "rule of thumb" I should use? That is, should each flash simply fire to get a "normal exposure" or should I dial back the background flash(es) to do only half-normal, perhaps?
No rules. You may want to overexpose the background a little, so as to get a white seamless really white, than the auto flashes would be set so as to overexpose, with plus corrections or lower iso setting. Or you may want to make the background look a little subdued by underexposing it a little, by the way of a minus correction or higher iso setting, or in a more interesting way, lighting it selectively by using zoom heads and highlighting small areas in it.
If I am going to do this, it has to be simple. In my typical situation, there isn't too much time for playing around, test shots, etc.
Mixing systems is a little challenging at first, but once you have studied it, it helps to have a few basic strategies ready.
Example: a really quick group shot in a large conference room. My plan is to start with an iso of 200, two SB-26 at manual ½ pwr aimed at the white ceiling and a camera-mounted flash (any model) set at autoA for a normal exposure, so that I can move around and maintain correct exposure whithout much readjusting. This will produce an acceptable shot of a lighted group in a lighted room.
snip: The rest is pretty easy: you can use all-manual, all auto A, or a
My question, I think I have already said, is what rules of thumb should Ii use for choosing auto or manual?
Do test shots in your house and a few typical locations and have scenarios ready for what works the best.
Chance favours the prepared mind.
Studio photographers like me, when they venture outside, will tend to use small flashes just like they're used to do with the strobes: all manual.
Automation, even the most sophisticated systems, require attention almost as much as when going manual. Even with iTTL multi-flashes systems, you still normally have to evaluate test shots, and type corrections on the master flash, instead of simply walking to each of the units. You may obtain the same excellent results maybe a little faster. Maybe. If there is zero time for even one test shot, then the automation may be Godsent! Automation really shines when in a social event you take a quick shot, then turn around 180° and take another shot of a completely different subject. In studio-type shooting on location, I only use it if prominent, recordable ambient light is constantly changing, or I have a flash mounted on the camera with me or the subject constantly changing distance.
There are "digital" slave triggers that will fire the mounted speedlight on a selected firing: these may be set to trigger on the third, fourth firing, therefore old units fire at the taking moment only. This takes care of the preflashes. These I have'nt a chance to study, but they seem to be an expensive solution. I'd would'nt bother.
Good luck.
JB
--
Jean Bernier
All photographs are only more or less credible illusions
] 
