again, using common sense as fa as possible. you can intercrop with things that don't mess up your rotation, like lettuces etc. or, if you do use a specific group, like radishes = brassicas, the effect should be negligable as it's a very short term crop. so it probably doesnt have much effect on the soil. (or so says joy larkcom)
that reminds me - should change my radish row to a new location. it's in the bean bed that will be brassicas next year so best move it along now....:)