I recently filled my 55g with water after adding ground peat moss as the first layer. It worked good in some ways but horrible in others. I put a thin layer of the peat moss and put a thin layer of pea gravel on top. Then, I added some water for my plants. However, the peat moss that was not covered completely by the gravel started to float. It was a big mess and it has taken it 3 days to clear up. But, in the areas where pea gravel was completely covering the peat moss, the peat moss stayed at the bottom (I can see from underneath the tank).
I think it is fine to use as long as you make sure to completely cover it with a substrate before adding water. That is my experience with ground peat moss. I don't know about un-ground type.