In theory if you use the smallest picture size (160x120) and are willing to wait several seconds, it may be possible if you are using a mega. In general, the Arduino is too memory limited and too slow to do this in anything like real time.
Even if you had a faster processor, say a 700Mhz Raspberry Pi, the limiting factor may be the speed of the serial download from the camera. Now, on a R-PI you could might be able read some of the 6 cameras in parallel, but it might depend on whether you need hardware serial ports to do the reading, and if the R-PI has multiple ones.
One way to solve the problem is to get 6 cameras that use SD cards, and put in 6 Eye-fi cards, and let these cards talk directly to your wifi host without going through the Arduino. Or get something like the Samsung SH100 that can be commanded via wifi, and do the uploads directly. Of course both of these solutions is probably several hundred dollars with the cost of cameras and maybe Eye-fi cards.