The HOHO buses in Rome are generally pretty weak. If you don't need the commentary, I'd not spend you money on them. They're quite limited in where they can stop due to new "traffic rules" implemented last October. For more info you might read: Hop On/Hop Off Buses in Rome,

The HOHO bus is a separate ticket than the ATAC bus pass that comes with the Roma Pass, So you cannot use your Roma Pass on the HOHO's. Thus, if you bought these two passes in the same time period, then yes, you would be overlapping.

You really do not want to pre-order the Roma Pass. They DO NOT send you a packet but rather a code with which you can pick up your "packet" at various designated locations, including Tourist Information Booths. The problem with this is your hotel or B&B may not be near any off these pick-up points and thus it becomes quite inconvenient. At the FCO airport, if you come in early, the P.I.T. (Tourist Info Booth) may not be open yet (It opens about 09:30). You can find the Roma Pass at many Tabacchi shops, newsstands, sites, museums, or tourist info centers. Some hotels even stock them... so better to wait and maintain your flexibility.

There are many buses you can ride that will show you the downtown area. The smaller electric buses are great - like Bus #116, or either #117 route: Bus 117 to Piazza San Giovanni in Laterano or Bus 117 to Piazza del Popolo.

The 492 bus will run you from the Vatican and up the Via del Corso to Piazza Barberini. The 81 bus runs from the Vatican to past the Colosseo. The 115 bus will take you up on Janiculum Hill for a great view. For more info on buses and bus routes, you might read this article: Riding buses in Rome - Good Luck!