But you can only do ONE first solo.
I politely differ; my fixed wing CPL only constituted a student pilot permit to fly the helicopter (helicopter is a separate license), so a signoff for first solo was required, making it my second first solo. This would also be the case for a helicopter pilot taking fixed wing training. A float rating also requires solo flight to complete the rating, and that requires the authorization of the instructor, though as it's still the same license, I'm not sure that counts the same way.
Though I've drifted the topic, sorry. Tailwheel flying, like driving manual transmission, is it's own reward in operating pleasure. Take whatever opportunity you can to gain that experience.