Is it okay to use an iPhone power adapter for other devices?

I have some other devices, Philips Sonicare Toothbrush, Xiaomi Powerbank, and Bose Headphones.

Can I use iPhone's 5W power adapter and/or USB cable to charge these devices? Is it safe for their batteries?

The power adapter is a standard 5W USB power adapter that functions identically to any other 5W USB power adapter. All USB devices have circuitry inside of them to turn the USB power into the correct voltage for their batteries, meaning it is perfectly fine to use your iPhone's power adapter with them.

The USB cable on the other hand, assuming it is the same one used to charge your iPhone, is a Lightning to USB cable, where Lighting is the proprietary connecter used on Apple's devices. This cable isn't compatible with your other devices, and won't even physically fit!