What exactly is your project? Using a serif for display and a sans for body text is non common. Frequently, it's the other way around. Even color and x-height are going to be issues mainly if you were to mix the two on a line, which you probably wouldn't be doing here.

Different projects might have different goals for each member of the pair. Sometimes, you want maximal contrast, so mixing a humanist serif with a geometric sans is a good idea, whereas sometimes you don't want that contrast, and using a pairing specifically designed to work together (like Scala and Scala Sans) might be better.

However, the basic criterion that it all comes down to is: do they look good together? The eye is always the final arbiter when making pairings like this.

Thanks for the pointers.

My project is creating a visual ID for myself, i.e. portfolio, business cards, eventual website etc... So there won't be great amounts of text anyway. And no, I'm not planning on mixing the two on the same line. At this point I do not have any sketches I wish to share.

I fell in love with Cisalpin when I realized how well it works both on paper and on screen. It's a neutral font, and I'm quite used to having sans as body text (I'm European, see). Whereas I see ITC Caslon most definitely as a display font (or at least I don't own such fonts from the Caslon family that would work well in body text sizes). So, I will definitely be using Cisalpin, the Caslon (/other serif/any display font) issue is still open.

My idea was to create a clean grid-driven design riding a sans workhorse, then add a bit of eye candy with the use of a delicious serif font.