"Now I take it that τρίβουσι χρόνον means "passing time", but I am not sure what σύμφωνοι is about."
Break it down etymologically, as in voices together. So the crocodiles will be..erm singing? whereas the kitharistai (not harpists!) let their kitharai do the singing for them. In a metaphorical sense. If you're willing to be gracious to the author you can assume he knew the technical/idiomatic sense of kitharizein to mean "accompany oneself on the kithara" and that he transposed that meaning to mean they're playing and singing.
You've got to be looser in your translating.
(And what does μελετάω mean? When I look it up it seems to mean "to care for, to attend to" but here it seems to be the same as μέλλω - "to intend to".)
I think here it basically means prepare/about/intend to. This seems to be the case elsewhere for the verb too, in Xen's memorabilia I believe.
"Oh, and μεγάλοι - obviously the crocodiles are huge, but does μεγάλοι κιθαρισταί mean "great harpists"? I.e. can it refer to quality as well as physical size?"
Yes, regnal coinage/titles etc seems to take it in terms of quality. See also Megas Alexandros a.k.a Alexander the great.