Unlike with USB 2.0, where connecting a non-Hi-Speed device or a USB 1.0 device can affect the performance of the entire USB bus, Thunderbolt is designed to handle multiple devices of varying levels of performance without affecting the channel itself. Of course, those devices still share the total throughput of the Thunderbolt channel, which could limit the performance of a particular device if multiple devices are transferring lots of data at the same time, but the performance of the Thunderbolt channel itself shouldn't be affected.

It depends. If you connect those devices to the of your Thunderbolt chain, they should't adversely affect the performance of faster "upstream" devices. If you connect non-Thunderbolt devices in the of a Thunderbolt daisy chain, the you connect them matters.

For example, if you use two FireWire-to-Thunderbolt adapters to put a FireWire hard drive in the middle of a Thunderbolt daisy chain, the performance of the rest of the chain "downstream" from the computer will be limited by the FireWire drive's FireWire bus, which simply can't pass data as fast as Thunderbolt.