are the custom firmwares specific as to which jailbreak software you use? for example if i download a custom firmware, can i use Redsnow or Pwnage to build the SW?
A custom firmware is pre-jailbroken, no need for any additional tools to jailbreak. You may however need one of those to prepare the iphone to accept the custom firmware depending on your situation.
If you are already on 4.1 there is no need to make a custom firmware. Just jailbreak with either limera1n/greenpoison/redsn0w. Either of the three should work fine.
custom firmware will probably be useful when upgrading from 4.1 to 4.2 but only time will tell. until now I would recommend using limera1n, its fast and easy.