Can I use Roma Pass from Termini to Ciampino Airport or should I buy separate ticket?
Yes, but not directly from Termini
The Roma Pass is only valid on the bus services operated by ATAC. ATAC operates two services to Ciampino Airport. The first service is bus line 720, departing from Laurentina Station on Metro line B. The second service is bus line 520, departing from Subaugusta Station and Cinecittà Station on Metro line A.
The Roma Pass is not valid on any other bus or train services to Ciampino Airport.