We chose to do Singapore 2a with our 7-year-old after Beast2A. We (parents) hadn't been diligent enough about assigning math so she was finishing up Singapore 1A IP when Beast 2A arrived. We bought the Singapore 2A, textbook, workbook and IP. With the plan of reading the textbook on the couch, then doing just the IP, then backing up to the workbook when/if things get difficult. Subtraction with renaming was difficult because she kept trying to do it all in her head - insisting on doing it in her head, and doing it wrong. So, we went back to the workbook for that. Also, it wasn't really covered in Beast2A. I suspect that is the only time the workbook will be needed. Just in general, I figure that a little bit below her level shouldn't slow her down much because it will take less time per page.
I am a little annoyed that I can't pre-order the rest of Beast 2.