If you have trouble setting the size/position of the CAS buttons you should check out this module, does all of that for you:
To make a custom mobile GUI you could do something like:
if not (UserInputService.KeyboardEnabled) or (UserInputService.GamepadEnabled) then --If player is on mobile
--Set up mobile gui
SomeMobileGui.MouseButton1Down:Connect(function()
Action() --Call action you want the button to perform
end)
end