I thought species like Skrit Na were class 2. If I remember correctly, Class 1 are hosts that barely qualify as viable hosts. Taxxons because they're so hard to control and need almost constant feeding. Gedds because they're weak in just about every area. Then Class 2 are species that would make good hosts, except for one very inconvenient thing. Like Skrit Na metamorphosis.
Class Three are hosts that are good, but too low in number. Class 4 are hosts that are good, but currently too powerful. Then you have the Class Five holy grail.