Can I have my main's score show when on my alts?

Yes!

Once you register an account on https://raider.io you can mark a character as your main. At that point, after the next AddOn database refresh your alt will include a reference to your main's score -- as long as your alt qualifies for inclusion in the database. This means you must have logged into the alt in the past 30 days, and have a minimum score of 200.

Additionally, if you are subscribed at Patreon Honored or higher, your alts will not be required to have the minimum score, making it a little easier to jump right in on an alt and start grouping.