1: Yes it should be. At least my AMD 2x5970 + 2x 5870 setup works ok. 11.12 catalyst is needed though. the new drivers work if I go to safe mode and disable one of the 5870s. So if you want to avoid driver swaps every time you do gaming then you should either dual boot or just use BAMT for mining.

2: You don't need adapters, just turn the secondary PSU on before starting your PC(primary PSU). You need to ground the green wire on the 24pin mobo connector of the secondary PSU to turn it on. I actually have a relay that does this for me when I start my PC. However some GPUs can be picky about the 6pin PCIe power not going up simultaneously with the power from mobo(that's why you might need to start the secondary PSU first).
There has been a lot of talk about this on this forum, and the consensus is that it is safe while it may not be practical at times.

3: If you can spare it buy the Antec it probably has much greater efficiency than your current one so it will eventually pay itself back. I do not recommend using those 4pin molex to 6pin PCIe power adapters as you are pulling the amount of current through 2 wires that you normally pull through 6. While it will work it is not good for the graphics card in the long run. The power components are starved of current and need to work harder. So either buy some weird splitters or make your own.

I made my own and use a spare 8pin CPU connector cable to power my gpu.