yeah I actually like the combination of farnahm for body and mercury for titling. Also caslon looks nice for callouts. What do you guys think about caslon and mercury . Sounds fishy but it actually looks good!
The convention is usually to avoid mixing similar styles. If the weights are different enough, or the style details are not similar, then I think it's easier to do without making it look like you made a mistake and mixed them unintentionally.