Is it permissible to connect 3.3V powered devices to the 5V I2C bus?
There are 3.3V devices that have 5V-tolerant inputs; that may be the case for your devices, but I would not risk it. Use a level converter designed for I2C, such as https://www.adafruit.com/product/757. Or you can make your own; https://cdn-shop.adafruit.com/datasheets/an97055.pdf explains how such a level converter works. Use a BSS138 if you are comfortable with soldering SMT components; otherwise use a 2N7000.