As far as I understand it you can use the Early American History Primary Pack anytime during the K-3 years. Some people will use it in year 2, others in K. The books are appropriate for any of those grades. This particular study does take 2 years. The other guides are written for grade 4 and up, but most 2 or 3 grade kids could understand and benefit from them. According to Beautiful Feet there is no real order to do the units.
http://bfbooks.com/Is-there-a-suggested-study-sequence-I-should-follow-with-BFB
If you started in K you would do it for 2 years, ending at the start of your grade 2 (or it could take longer if you add in other resources, movies, field trips, activities, and such). You could then do whatever you felt like….adapt the intermediate level, adapt the ancients, use the SCM guides, do the BF character study or the history of science.