“No son unos salones cualquiera”
“No son cualquier salón”
Cualquier = apocope of cualquiera.
It is an undefined adjetive which can not be in plural.
In the first sentence, the objet “salones” is plural because it follows the verb “son” which is plural.
In the second, the objet follows the adjetive which is singular so it has to be singular.
I hope it helps you