O.K. here we go: the proof of my statement above:
Sony A700 used with on-board flash
to trigger an expensive Metz 45 CL3 flash and an extremely cheap Centon F.G.3.0.D flash (somewhere around $10). In the menu of the A700 I set the flash to manual. Regarding Springtide's comment: If you set it to pre-flash ttl you can trigger the Metz flash but not the cheaper Centon. In other words if you have no manual setting in the A300 or 350 there are still a lot of flashes out there that you can trigger with pre-flash ttl!
Please alsosee the little slave cell attached to the non-Sony flashes!
This time I used a HVL F58 AM flash and not the on-camera flash to trigger the other two flashes. The Sony flash was attached to the hotshoe via the mentioned cable and you can see it in the foreground to the left. This time the camera
was set to manual priority instead of A or S and in the menu the flash was set to manual finally I also set the Sony flash itself to manual. So set all you can to manual
So you see this works as well.
Now to last points:
a) before you ask how to control the output of the different flashes used. Well- stockings; bounce of walls; white plastic bags; umbrellas; bounce of white/coloured sheets of paper; put baking paper in front of the flash and last but not least the most important: you control the output by the distance of the flash to the subject you photograph and often you can control the output directly on the flash i.e my Metz flashes can do that the Centon as it is too cheap can't
b) wireless flash
CAN NOT be used in connection with non-Sony flashes BUT considering the fact that there are tons of cheap flashes out there and the slave cells cost you around 15$ this is the much cheaper version and frankly I have 5 Sony flashes and around well plenty of non-Sony flashes including three Metz. The wireless system does not work as well in my opinion as the slave cells. Each time I find a cheap flash I buy it immediately - together with a slave cell.
If you look carefully at the pics- yes the Metz is set to 1/16 of it's possible full output while the Centon is set to full power but no too bad for such a little one- don't you think? It's the perfect output for highlights on hair just to give an example.
Hope this shows how it is possible to use non Sony flashes with Sony cameras and I really hope that the one or other of you will give it a try as it is the cheapest and I think most effective way to get into studio photography!