Hi @Ellie_moor, first welcome to the community!

Yes! You can always create your own Segment to be used as the target audience - or in your case - as the excluded audience that has more specificity of whom you want to target (or not target) the Signup Form to appear for.  

I think the “Don’t show to existing Klaviyo Profiles” is just a quick/convenient way to not show the popups to any identified/known Profiles without first creating a Segment - it is by definition all encompassing catchall for only new prospects. 

There’s no real drawback, but make sure you think of all the edge case scenarios for that Signup Form.  For example, a user that has unsubscribed (or chosen not to subscribe) will still see the Popups unless you specifically add those rules to your Segment. Also, make sure to include other “Lists” that you may not want to see the popup - in the past, some of my clients didn’t want popups to show for “Wholesale Customers” or “Staff Members” (shouldn’t they be subscribed?!) since they are on the site daily. 

Hope this helps!