Only you can know your goals, but I'd really encourage you to do the science in Bigger in lieu of the other science, as all of Bigger's science links to the history and has so many wonderful and necessary skills integrated into it that are pretty tough to teach well on the fly, as they are guided/taught so well in the plans:
Each unit includes the following science activities in coordination with the
read-aloud assignments:
*Day 1: practice narration by retelling the science story
*Day 2: create a science notebook entry
*Day 3: conduct an experiment related to the reading and log it
in a science notebook
*Day 4: practice narration by retelling the science story
*Day 5: conduct an experiment related to the reading and orally
discuss it
Here is a recent thread on Bigger's Science that may be of interest to you:
viewtopic.php?f=6&t=5797&p=42536#p42536
With all of this said, if you are still desiring to do just some of the science in Bigger, it is all linked to the history well, so I'd base it on how many units a book is used. OSS Seashore is used from Unit 1-Unit 5; Science in Colonial America from Unit 5-Unit 9, John Audobon Unit 10-14; OSS Woods Unit 15-18; A Pioneer Sampler Unit 19-31; Thomas Edison Unit 31-34
I'd really encourage you to give the science in Bigger Hearts a fair fun though, as our dc have learned to think scientifically, love science, and score well on standardized testing due to the way it has been taught in HOD.
In Christ,
Julie