A good one is Legendary Items, it has a large amount of items.
Yes, it is possible to download multiple item packs at the same time. You just got to add the things from the items.cfgs to the bottom of the first items.cfg you downloaded and used... otherwise you'll just end up with the items from the last item pack you installed. Though if someone else has done the same with different items packs, you might see some strange things. xD