It is said that during Brahmotsavam, our ancestors come down; at this time can we worship Divya Prabandham in a goshti with non-Srivaishnavaos? Will it beget any sin?

No sin will accrue when Divya Prabandham is worshipped in a goshti with non-Srivaishnava-s; but each temple follows a custom so one has to know from them and worship accordingly.