Do it! Im not sure if you feel comfortable putting something from the canal in your tank, but they are pretty much ready to go when they are floating around like that. Should start to poke out a root after about a week of standing in the substrate. Dont expect to see true leaves for a month or so. I have a small tree in a ten gallon 'fuge and it can denitrify 20 ppm in that much water in two days. I have read that after a while you may need to add magnesium but I havent tested for that yet.
Good point about needing alot of substrate. Substrate would need to be at least three inches to start, with the dark end down far enough so that it wont come out, and the top has to be out the water.
Changes in salinity dont seem to effect it much, if any. At least at levels less than 30 ppt.