By varying paper weight from about 50 # to 300 #, I have found that the best paper weight is at least 100 #. You can use 90 #, but there are problems with it tearing during processing.
Baryta paper, DW is subject to causing imperfections due to its 'hardness' and also thinness. SW is virtually impossible to hand coat on.
Multi ply papers coat fine, but often process poorly. COT 320 is very good, but difficult to judge which side is sized and it seems to vary between batches.
You can coat on almost anything.
Impermeable coating substrates require different formulas than permeable substrates. BE AWARE OF THIS PROBLEM..... There can be a big change in speed and contrast.
You do not always need a sizing layer.