I know how to edit and create costumes, but how do I;

a) change body specs such as height, etc.
b) display kits, such as body armor, etc?

Thank you!

you can't

must be in starfleet uniforms (not civies) and only a few kit parts are available. No armors on starfleet characters.

Klingon have armor but it's not the type associated with item-armor - it's cosmetic.

I still can't find the option to add kits to the costume...

For the most part, there isn't one. I think there may have been a couple of options when the Foundry was on the test server, as well as the option to add cargo pants pouches to any uniform. (I miss the latter.)

Pars of just science kits can be added, afaik.

No tactical or engineering kit parts are available.