Grid Row Layout - multiple components

A stacked multi-component single cell is not supported in editable grids natively.
Use a!forEach() with a!columnLayout() instead if you need the text field and rich text below it in the same row(mimic a grid).
or
Use a!textField with instructions parameter - it shows helper text below the field, styled smaller(Most Probably will not show but  give a try).
That's the closest native option inside an editable grid cell.

No this won't be possible unless you build a custom grid entirely using various components instead of a!gridLayout(). When using a gridLayout, you can add a separate column for the content expected in Rich text field component so its in the same row. 

Hi [mention:878ddaf1e8634c529ae875ad8a4465e4:e9ed411860ed4f2ba0265705b8793d05] 

You cannot achieve this using an editable grid but you can create a workaround by creating a grid using the card layouts and custom pagination some thing similar to what is suggested in this post.