Toprol is an L3 - which is tested and safe in lower doses (50-100mg twice daily). For the first 3 months, just keep an eye out for a slower heart beat. Babies will have a very fast heartbeat, and it's something a Mom would notice anyways.
Synthroid is an L1 - the safest level.
L1 = safest
L2 = safer
L3 = moderately/mostly safe
L4 = possibly hazardous
L5 = DO NOT TAKE
Medications and Mothers Milk (and pregnancy) by Dr. Thomas Hale 2008
As for Baby #1... he nursed so much in the beginning to get your production going to the supply needed. Also, they are trying to get the hang of nursing and are going thru so many developmental stages the first 3 months and every 3 months after. Don't get discouraged about baby always wanting the breast - it's normal and if you supplement at that time - you will sabotage your own milk supply.