Would it cause any harm to the battery to use a Torch charger for the Playbook?
No it will not cause any harm, just be aware that it will take a lot longer to charge. The PB charger has a lot more power than the standard BB phone charger.
As stated above I would use the PlayBook charger to charge your phone more so than your phone charger to charge the PB. There was a forum on this about a week and a half ago. The amps from your PB charger won't hurt your phone, and if you charge either of them with a car charger they will charge even faster. Do a search for the reasoning, someone went in to great detail about it.