Yes, you can play against bots in Co-op mode. I've been playing for a few weeks, generally 2-3 games every day, and just hit level 20 this morning. Co-op is usually quite easy, and in my experience, players tend not to say much. Occasionally you'll get someone being rude, but it's easy to put them on ignore.
*edit* - Also, you get extra exp for queuing with a friend, so it's still a good idea to find a buddy to play with!