You can change this behavior in the ‘Composer Settings’ panel. Open the panel by clicking in the Composer window to make it the active window, then either press Ctrl+= (win) or Cmd+= (mac), Right-click, or Ctrl-click in the timeline window.
The Composer settings panel has several tabs. Select the ‘Edit’ tab, and look for the checkbox called ‘Auto-create New Tracks’. Turn it off. This will prevent avid from automatically creating new tracks to accommodate whatever you’re adding to the sequence.
In the settings tab of the project window, you can duplicate your composer settings file, and switch between them simply by clicking in the space to the left of it. You can also use workspaces to automatically switch between groups of settings, so that if you find you like things set in different ways depending on the editing task at hand, you can use keyboard shortcuts to swap your settings instantly.