These are the rules of all- as a pronoun:
aller m, alles n, alle f, alle (pl.)
- (indefinite) in singular without a noun (usually alles): everything
- (indefinite) in singular with a noun: all (the) (+ singular noun)
- (indefinite) in plural without a noun: everybody, everyone
- (indefinite) in plural with a noun: all (the) (+ pl. noun) Examples: Menschen aus aller Welt = people from all over the world (die Welt, pl. die Welten) Alles ist gut. = Everything is fine/okay. Ich wünsche dir alles Gute. = I wish you all the best. Jetzt ist er aller Verantwortung frei. = Now he is free of all the responsibility. Alle waren total verständnisvoll. = Everybody's been so understanding.
It seems a bit paradoxical to talk about "alle" or "all" in english as a singular element since it refers by definition to an entire set of various elements. However, if you consider it as the "whole", this is, the set containing all of the elements, then you're talking about one object still, so the singular is the proper form.
Both can be used. Check this site: http://www.german-grammar.de/grammar/chapter_12/12_11_2_jeder_alle_as_noun.htm ;)