You can specialize in more than one topic without losing credibility with your readers. In fact, you may find they are glad to see random topics mixed in with what they are used to seeing you write.
If you want to succeed in freelance writing, variance can be a very good thing. Clients love writers who specialize. But they also like to see some versatility. This way, if they have a topic that shies away from your usual routine, they know they can at least consider you for the project. However, if you only write on one topic, how will they know if you are able to handle anything else.
Are you showing enough variance in your work?