Can I write a Song of rhythm patterns that have different time signatures? If doing so, do I need to take any specific steps with the time signature of the Song?

The manual does explain this one pretty well. Check out "Setting the time signature" (under "Before you start to record")and then esp. "Changing the time signature during a song" (in the explanations for microscope edit); you can change the time signature virtually whenever you like on the "beat track."

Forgot: When using patterns with different time signatures, in the measure where a given pattern is inserted, you also have to insert a time signature in the *beat track* that is the same as the pattern time signature, since the beat track time signature takes precedence over the pattern-track time signature. The manual explains this as well.