In the second movement, for about a couple of minutes, starting around the 2:20 mark, one of the violinists plays, the cellist plucks the strings of the cello(pizzicato?)...can that "part" of the movement be classified as 'Siciliano'? I realise that it's usually a whole movement, but I'm just wondering, since I feel it has a certain amount of "Italian-ness" to it.

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Regards,
Navneeth

The closest thing to that quartet I have handy right now is Mahler's orchestration, somewhere in my hard-disk. So I'm not going to do an aural checking of the part you mention.
However, I don't think there's any problem if you call it siciliano. That rythmic motif: long-short-long, can be considered just as some sort of texture, so then... you can have sicilian expositions throughout a work.
Just like the fugues, they are a texture you can use whenever you want.