My bro, who is currently in Singapore Polytechnic says that students who took A levels in Malaysia might have to take SAT if they want to enter NUS. He's going to NUS after polytechnic, so they don't require SAT. Instead they just expect a 4.0 cgpa from you.

You can go to polytechnic right after spm, but it depends on what type of course you are pursuing. My bro is studying aeronautical engineering, earning his diploma and will be entering NUS after 2 1/2 years. He entered SP right after SPM. His friend is doing the same and he is studying banking and finance