Hi Doni and welcome to the forum. Personally I haven't tried making mozz with skim milk, but I think the addition of calcium would definitely help with curd formation.
What ever rennet you use, just use as directed on the packet. Cheese Head has an analyse/explaination of liquid rennet somewhere on the board, and from what I can remember, it is a man made rennet originating from animal rennet. Someone please correct me if I am wrong in my understanding.
Having said all that, I can only get liquid rennet here, so that is all I use, and I am happy with the results.