No, it doesn't have to be rednand, it could be MochaCFW, or Haxchi, or CBHC.
And yes, you have to load the CFW every time the system boots if you want to play DLC you don't own. Or you can use a cold boot custom firmware solution, such as CBHC. Then the custom firmware will be loaded when the system boots up.
If you try to load a dlc that you don't own without first loading up your CFW, the DLC just won't load in-game, or it may give an error message. That's it really, you shouldn't brick or anything major like that.
(Sent from my Samsung Galaxy S6 Edge+ using Tapatalk)