I've read that there are 4 distinct although related forms of AGs : Seyferts, Quasars, Radio and Blazars. Are there any other forms possible? Also do these forms ever intermix to give something new?
Yes of course, there are other forms apart from the four stated above. Basically you have just two kinds:
But then again, there are two Seyferts
Apart from Quasar, Blazar and Radio galaxy, we also have presently,
Best source to begin with is wikipedia.org of course.