[SalesForce] How to customise columns in Add Products popup

What you can do is create a rollup summary on the Opportunity that counts the number of products that are added to the Opportunity. Then, with validation rules, you could prevent the progression of the Opportunity Stage to anything other than the default stage when that count field equals zero.

In your validation rule you can also check the $Profile and $User globals to restrict only certain profiles or User attributes.

Edit Update: Updated the answer as per Vamsi's suggestion in the comments i.e. no need of a controller and can be achieved via VF page.

Create a new VF Page:

<apex:page action="{!URLFOR($Action.Product2.List, $ObjectType.Product2)}"/>

Go to Customize->Products->Buttons and Links

Edit the "Products Tab"

Under the Override Properties section, for "Override With" property , select the visualforce page name which we have created above.

We have now successfully overriden the Products tab. Whenever you click on the products tab the user would be redirected to the URL which ends with '/01t'.