Yes, "es" is in this case only used if there is nothing else in the beginning of the sentence (the so-called Vorfeld).
There are actually four functions of "es", as outlined in this exellent answer:
expletive (placeholder filling the beginning), referential, impersonal (placeholder for an object) and correlative (placeholder for a following clause). The "es" in "Es sind" is expletive, it is a placeholder for the beginning of the sentence and it vanishes the instant this beginning is filled, for example by moving "Am Himmel" into the front.
Es sind Wolken am Himmel. Am Himmel sind Wolken.
This is different from the "es" in "es gibt", which is impersonal (placeholder for an object). It can't be left out.
Es gibt Wolken am Himmel. Am Himmel gibt es Wolken.
Note that "sein" ist also used with the impersonal "es", but with another meaning, with distances.
Zum Bahnhof sind es 10 Kilometer. Bis Weihnachten sind es noch 10 Tage.