I can add to @anon35827816 reply that I would like to see the full sketch of the Master and the Slaves. I assume that the ESP8266 is the Master ?
Can you show a photo or schematic of how the level shifter is used ? It probably has already 10k pullup resistors on both sides. You need to apply 3.3V to the low side and 5V to the high side, or else the level shifting does not work.
Seven Arduino Nano boards with internal pullup resistors is no problem if there is nothing that makes the signal weaker. A level shifter makes the signal weaker, but I think it should be possible to make it work.
I wrote something about pullup resistors at the bottom of this page: How to make a reliable I2C bus.