At 7 weeks pregnant, my identical twins both had a BPM of 180. At 8 weeks, it went down to 154 and 168. Is it okay that the BPM went down?

It's okay. Congratulations. It is normal for fetal heartbeats to vary up and down quite a bit, unlike an adult's heart, which is steadier. This is because the fetal nervous system works differently. Your babies will have different heart rates--that's normal, too. As they progress, the heart rate may go lower. In the third trimester, 120 BPM is not unusual.
Size vs BPM. Throughout all animals, there is a general pattern of things. Small things tend to have faster heart rates than larger things. This holds true as babys are growing as well. As babies are growing and developing, both before and after they're born, their heart rate steadily decreases to a normal adult rate when they grow up. Perfectly normal.