Great question and one i wish ididn't have to answer. But hey there is always one trouble maker in every crowd
The Answer to all your questions and i am pleased to tell you YES.
Now the hard part of the answer How?
Firstly as to an idividual trying to id the scat from similar type animals (have good references) they do have distinctive features and sizes. In saying that it could be confusing, say if If a young kangaroo dropped scat, and so did a large wallaby with similar features did in the same area, the scat alone would be impossible to tell apart. in this case you would look for other signs or traces, A good knowledge of species located in area is vital ( your Gov. Enviroment dept. may have lists).
One sure way is to observe the animals and when they leave the area collect fresh scat taking vital notes on collection details.
Ok With the last question telling species apart, other than info i gave above I believe you will be looking at more scientific research "Microscopes". it is possible even without DNA most animals do lick themselves in grooming and this often leave hairs in the scats. there is a book available on hair Analysis of Australian animals and I can't remember the title of hand , besides that it is well out of my ability to try and go this far into ID.
I hope this is of help I know I am not the best writer in fact hate it. But please ask if I did not make myself clear.
I will try and show samples of similar species in a different thread. Sometimes it is just impossible to tell
Really fresh wallaby and kangaroo scat is green and looks wet, at least in the early morning when they are still actively feeding. It turns dark and loses the "wet" look after the sun is on it for a few hours, and then dries out over the next few days to a paler colour (depends on rain, humiditiy, season and location). When checking a scat, don't be afraid to tease it apart with a stick to see if it is fibrous or not. This can be useful to differentiate small carnivores from small herbivores.