When you see something like (n) that means you can use an optional "n" in that particular position (but only "n"). Or (r,l,n) means you can use an optional "r" or an "l" or an "n" in that position, but not any other consonant. But (C) means you can use any consonant, including nasals, liquids, stops and so on in that position. r, l, m, n, etc. are all members of the set of Consonants, C. So no, they don't have to be specified seperately if you mean "all consonants."
But you do see them specified seperately a lot, because consonant clusters that include a nasal or a liquid are allowed in more languages than consonant clusters that are any two+ consonants thrown together. Like, more languages will let a word start with "pra" or "bla" than "pka" or "bza."