1- is there anywhere online considered to be THE how to guide for this?
2 - is this technique considered only for growing a carpeting on the substrate?
3 - I see lots of people talk about HC and glossostigma - what Other plants work?
4 - are there particular substrates that work best for DSM?
1- Dont think there is one particular "THE how to" out there, best to search for similar examples for what your planning to do
2- For the most part for many that do it, yes, i'd say so. But not limited to just carpet plants
3- There are lots...lots of aquatic plants can handle it quite well. There are a few that wont, like blyxa, vals etc.
4- I would recommend Aquasoil personally, but any nutrient rich substrate should in theory work the same