Beauty and Truth Math & SCM Elementary Arithmetic

The Strayer Upon Books indicate Book 1 is 3/4th grade, Book 2 is 5/th/6th grade, and Book 3 is 7/8th grade.  Two of my children are rising 3rd and 5th graders, so I could potentially just start at the beginning of those books, right?  But the Beauty and Truth CM guides for this series indicate a slightly different start time.  For example, Book 1 is started in 2nd grade and Book 2 is started in 4th grade, if I recall correctly.  I do feel my rising 3rd needs to go back and work on understanding multiplication from before the 6 times table, and my 5th grader could use a review in operations.  They both used Singapore Dimensions last year, which does sort of review and add onto concepts from year to year.  I like it, but I feel like the S-U books might be easier for me to teach.  But if we are going to use these neat guides, the guides have a way to incorporate geometry and algebra, and we’d always be “off” the schedule, I guess.  🙁  Has anyone switched and had this dilemma?

I have also looked at SCM math, at least for the 3rd grader since they only have Books 1-4 out.  Would Book 3 be a good place to start if she needs to be introduced to multiplication again?  I really like how CM constructs times tables.

Is this math rigorous, should a student intend to go into any math/science fields?

I do not want to go backwards per se in their math.  I suppose we could stick with what we’ve been using, of course.  Or I might switch the 5th grader to an online math.  But thoughts on these?