Youre 100% good to go. I got confirmation from Cast themselves when i asked the same thing. Their response from an email i sent last march:
“Thanks for reaching out. Yes you are correct. You can use any of the Pivot heels with the 2nd ski Kit. Its just the toes you can not use. You will just need to share your Pivot 15 or 18 toes with the 3rd kit with P14 heels. Let me know if you have any other questions?”