Google's John Mueller said in a live Google hangout on Google+ at the 13:05 minute mark that serving multiple types of mobile friendly options on a single site, works fine still.
He said that you can mix using responsive or dynamic serving or separate mobile URLs. He said just do it on different parts of your site, so your blog versus e-commerce vs main informational part of your web site. He said, being mobile friendly is more important that what flavor you use to become mobile friendly.
Here is the transcript:
So if if you use responsive or dynamic serving or separate mobile URLs, it is essentially up to you. It is something that you can also mix, where you say well part of my website is fully responsive and part of it just sniffing user agent to make sure that we can like serve them properly.From from my point of view I just really make sure that everything works on mobile as well.
Here is the video embed:
Hat tip to @glenngabe for spotting this.
Note, the Google mobile friendly boost is coming next month in May.
Forum discussion at Google+.