Questions - I don't know the answer, but here is what I do know:-)
I purchased Singapore 5 A and B for DD in fifth grade accelerated in math to 6th. The book was too easy. I should have gotten challenge problems. I have heard however, that starting from 7th grade, all the challenge and word problems are already built into the original text.
Best way to go would be to find someone who is using the books in you area and have a good look at them.