Hi ya,
I usually try to minimize water transfer from other tanks to mine, but then after more thinking about it, I don't see what the big deal is unless there are major contaminants in the bag water (like phosphate, which is hard to remove from water and prone to give algae problems). Anyway, I would think adding water to your tank from another tank would increase the biodiversity of plankton in the water, which would help corals. I guess though if you have fish, you could be introducing parasites/bacteria, which is bad.
As for the ammonia, that is a tricky one. I don't know the answer, but I can say I put a med-sized tree leather in a small specimen container once that had several strands of macroalgae in it and it melted away in 5 days (I was away, and a clueless person was taking care of my tank). IMO, the leather produced a significant level of ammonia (granted small, but over a few days, it was enough to kill it). To be safe, I would say leathers do produce ammonia. It's safer working knowledge.
kris