I don't think that will really help. If you'd like to use an additive, Melafix would be more beneficial than extra water conditioner (I assume your
LFS person might be talking about Stress Coat... unnecessary product altogether IMO). Otherwise the best thing is super clean water like I mentioned.
I'm not sure how long it will take your barbs to grow their fins back, but I've had some pretty remarkable experiences with fin regrowth myself. For example, I recently bought 5 pygmy hatchetfish, and their caudal fins were all pretty ripped up from their LFS tank mates. I brought them home and did some extra water changes (my water changes are always 50-60%), and I've seen incredible growth just in the past 2 weeks. 3 of the hatchets look like their caudals were never nipped at all.