Catacombs in Rome

We went there on a small-group tour, but once there, had to go with a tour with one of the in-house tour guides. We had other sites included on our tour, but if you just want to see the catacombs, there's no real advantage to going on a tour with an outside company, other than the transportation.

As above, you cannot visit a catacomb without a guided tour.

Yes, you can join a tour at the sites but see the websites below for any pre-reservation mandates, such as at Priscilla. San Sebastiano doesn't require pre-reservations; you show up, buy your ticket and wait for the next tour in English. It doesn't look like Domitilla requires them either.

These tours are not expensive; abt. € 10,00 per adult. You were probably looking at independent-company tours which visit multiple catacombs and include transport. You could take a taxi or a bus. We reached the one we did (San Sebastiano) on foot as it was along the way of our Appia Antica trek. The websites below provide "How to get there" directions, and do check them for hours and closure days: San Sebastiano and San Callisto are close enough together to easy walk between them, if able to visit them on a day both are open.

Links to the websites of the most visited catacombs:
https://www.catacombesancallisto.it/en/index.php
https://www.catacombepriscilla.com/home-eng/
http://www.catacombe.org/uk_index.html
https://www.catacombedomitilla.it

Any remains which may once have been visible have been removed from the tour routes out of respect for the deceased. Photography/filming is not allowed, and respectful dress and behavior is expected 'cause they're considered sacred places.