i'm pretty new to bass (came from guitar), so bear with me.
I started playing on a freinds ibanex gsr200, i felt really confident with this bass, it had a thumb rest and i got very comfortable with it.
Recently i bought my first bass, i was comfortable with his so i bought the same model, mine is newer and features the active electronics. Unfortunately it doesn't have the thumb rest. I asked the guy in the store (sam ash) about purchasing a thumb rest, he told me it would mess with my pick ups and might ruin the bass

i'd really like to have one as it helps with control of my plucking hand.
Is this true, that it will mess with my pickups?
How can 1 gsr200 be fine with a thumb rest but mine would be messed up? (i was going to put my rest in the same place as my friends)
Is it because of the active pick ups?