The B777 is just too large and heavy to be operated economically permanently on short routes to CGK, KUL, PEN and others. They fly hourly between SIN and KUL, so I don't think they could keep these frequencies with an aircraft that is as large as the B777.
And the B767 doesn't make sense as well. It would be a completely new type in SIA's fleet. That requires pilots trained, cabin crew trained, maintenace crews trained, spares bought and so on.
The A330, if -200 or better -500 wouldn't be a completely new type as SIA already operates the A340. As those go, the A340 flight and maintenance crews could very quickly be trained (just an update), and many spares could still be used. Additionally SIA could use the RR Trent which I think is not available for the B764. A330s for A340s would be a smart move and would help to save costs. Not to forget that the A330 can take more and larger containers than the B767 and regarding the new Star Alliance cargo alliance freight is a very important issue.
And what about range? The A335 will have even more range than the -200 which means a great flexibility of the plane. It can be flown to KUL and the same day it can start to operate a flight to Europe. An option which is not given by the B764 whose range is a joke compared to the A335. I think flexibility is a big issue for SIA.
The only reason for them to order the B767 is to save the relationship with Boeing after the big A3XX deal...but I don't really think that SIA takes that into consideration while trying to get the perfect aircraft for their routes.
Oh, B757s? No way, SIA wants to keep its level of service and that is much easier by using a widebody aircraft. And reagrding the B757's cargo performance it even falls more...
Me & You & a Plane Named Blue...