I would definitely start from the assessment, and pay attention to any items they get wrong, because it may be that you will be able to give them a quick lesson on one or two points, and then move to the next level. I would not worry too much about coming in in the middle. There is a certain amount of slow spiraling in SM (although always at a qualitatively higher level). I started with 2B for my #1, and k for my #2. I did notice that it was easier for #2, having previous familiarity with the teaching style, but in terms of gaps, there was no issue starting with 2B for #1. (Plus, I think #2 is more naturally math-minded, anyway.) My sibling's child, if I recall, began with 3B or 4A, (coming out of a very traditional drill-and-kill curriculum) and settled into it after a brief adjustment period.
One of the patterns I noticed about Primary Math was that every grade started off with place value, which allowed for a very brief review of the basic arithmetic taught up to that point, mixed in with increasing amounts of mental math, which helped to keep the review-ish material from being boring, as the mental math tricks were kind of fun (if you like math tricks, which I do). Of course, these topics are placed there largely to combat summer losses; since we school year-round, our kids usually zipped through the first couple of units.
Another aspect I like is the subtle review of skills through the following several units, by writing practice problems that are ostensibly for a new topic, but have review of past skills woven into them.
All this to say, although of course it is always easiest to begin and end with the same curriculum, I think that, after the initial adjustment period (mostly to the pedagogical/problem solving approach), it will work out fine to begin with the middle grades material.
And to an earlier post: I would agree that it's possible to use only the textbook, as it has problem sets in it also, just not the occasional little fun puzzles or coloring math pages. You just have to use your own paper; my kiddos liked the workbook style.
