How to Combine the Simplistic GUI Pack with Other Interface Mods
Deciding What Each Mod Should Control
Interface mods often add their own buttons, panels, or HUD widgets. Simplistic GUI only replaces vanilla textures, so it will not restyle custom screens added by other mods. Plan which look you want for vanilla menus versus modded menus before stacking packs.
Ordering Packs and Mods Together
Put Simplistic GUI at the top of the Selected list so it wins conflicts on vanilla GUI files. Mods that ship their own textures load independently of the resource pack list, so they will keep their own appearance unless a pack explicitly overrides their files. If a mod's GUI looks out of place next to the clean style, look for a companion resource pack for that mod and place it just below Simplistic GUI.
Testing After Changes
After adding or reordering anything, open a vanilla screen, a modded screen, and your HUD in turn. Note which parts look flat and which still look textured. Adjust the order until vanilla elements match the Simplistic style. Keep a note of your final order, because adding a new pack later can shift priorities and undo your setup.