Question regarding ground when using teensy to control a mosfet switch
First of all, your circuit idea does not work. You are using enhancement-type NMOS (as source followers) to provide 5V from a gate drive of 3.3V. That just won't work.

Try either switching load ground, using PMOS (with some additional circuitry), or an integrated load switch to do the switching.

Now, to your actual question.

Assuming that all of your DC-DC converters are of the non-isolated type, yes, it will create a ground loop. But it might be a necessary one.

Given power consumption you will not really know what is the potential between the Teensy and your switches without directly connecting them together. This does mean that some of the ground current will be circulating through the Teensy.

You can avoid this by either:


Using isolated DC-DC converters and connecting the grounds together.


or:


Using isolators (opto or any other kind) to drive the gates of the mosfets.


You can also manage transients in the ground loop currents by adding common-mode chokes to the Teensy power supply.