The basic rule is not to combine two type faces from the same group on the same page! Also avoid using script and italic together. The theory is that if the difference between the type is obvious, then it at least looks intended! If the type style is similar, then it looks like a mistake! So if different faces are used, they should be obviously different. To choose wisely, we need to know which classes of type we are combining.
Rule of Type
Do not use
On the same page
The above type does come from different classes, so may not be too bad. The first is modern, the second is sans-serif, the third is oldstyle and the last is script.
Web Creation and Design --> Resources --> About Type --> Rules of Type Selection