Even though the 777-100 would be smaller, I would have thought it still too big for SQ. Already they have said the A332 is too big and even with a shrink the 777-100 would still be around the same size as the A332.
When you consider SQ want to replace A310s, A332s or 771s seem a big step up in size.
On a separate note, I doubt QF would be interested in the 771 in the short term. This plane would replace the 763 which tend to be the youngest part of QF's fleet. Replacing the classic 747s is highest on their priority list followed by replacements for their 733s.
An Australian airline which would look at the 771 is AN. Had it not been for the continuing ownership struggles (now resolved) AN would have ordered A332's a year ago. Should this plane become a reality it would offer an alternative to the A332. AN apparently consider 772s and A333s a little too big though they will probably happily take SQ's older 772s as a stop gap measure to replace some of their ageing 767s.