Can I write coding both traditional c# monobehavior class and ECS in a game at same time especially for UI design?

all coding via ECS is enough to develop a full game development?
,all code via ecs for a full unity game development is possible?

the mono behavior class IS a built in ECS system as I see Wikipedia is describing it.
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scripts of monobehavior MUST be attached to a specific gameobject (or child of a gameobject) and act as custom components. also monobehavior scripts can be added or removed to any gameobject during runtime in a scene using addComponent(“name of script”) to affect how any object acts.

the default calls of a monobehavior class is to itself when you use “gameobject” so if you want t use it as an “Entity component system” its easily applied to anything in the scene.

the wonderfull thing about monobehaior is it can also address anything else in the scene… or nothing in the scene as a masterscript!

So in short, unity is allready based on a Component system. whether you are adding custom mobehavior script components or unity’s built in components (such as rigidbodys and animators) its all about adding components.

if there is something specific you are wanting to accomplish let us know