Instrument inputs are for connecting an instrument (pickup has higher impedance)...
I would reserve the 4 inputs in the front for connecting Mics and Instruments.

On the back you have 8 analog inputs and outputs which are for connecting devices with line or studio level.

So a pedal or preamp you connect best via the analog ports AN 1-8 on the back of the unit.

Should also be clear from reading the manual what input is for what purpose.

It should be clear that an Instrument input is primarily being used for connecting mainly instruments with pickups.
Based on that .. if there are special instrument inputs for connecting instruments directly ...
well then the "rest" like effect, pedals, etc goes to normal analog ports ...

You only need to pick the correct line level (TM FX: tool/wrench symbol of your analog port).
Usually the consumer devices use consumer level. See manual.