The CDR rating is how many amps the battery can discharge safely. The amount of amps being used is a direct result of whatever power setting you set your mod to. So a higher CDR is always safer. HE4, HE2, 25R, VTC4/5 are all great batteries.
According to kidneypuncher, the iv mini 2 will draw a max of 25 amp from the batteries. Many would say that this means the vtc series are the only safe batteries to use with this mod. But I don't think you will have problems with any of the above mentioned batteries.