You are welcome to mix Backend Layouts and Grid Layouts like you need it. Please refer to
Columns width ¶
You can setup the with of margin and main columns supported by the Constant Editor.
- [START - TEMPLATES - GRID - 2 COLUMNS]
- [START - TEMPLATES - GRID - 3 COLUMNS]
Fluid Templates ¶
Use your own fluid templates. See Constant Editor > START – TEMPLATES*