Is it important to read the other books in the Giver Trilogy and is there any available research information on the other books in the trilogy? Would it be better to compare the Uglies to the Giver by itself? Should I pick a different topic altogether?
The three books in The Giver trilogy are really only loosely a trilogy. They have similar themes and some possible character overlap, but they do not have all of the same settings and characters. I am not saying that they cannot be considered a trilogy, but they cannot really be analyzed as one in my opinion. I would compare another trilogy, such as The Hunger Games. It also is a dystopia and has similar themes.
I think that your topic is fine -- no need to go for a different topic.
I think that you do not need to read the other books in the Giver trilogy. I do not think that they are nearly as good and I don't think that they advance the themes that The Giver and the Uglies have in common nearly as much as The Giver does. I would just stick with The Giver and the books of the Uglies trilogy because I think that the other Giver books don't add enough to make it worth bringing them in.
I think that you have a viable topic for analysis. One convergent theme in both works is the idea of a utopic society. Both works really delve into the idea of a utopic setting and the dangers inherent in it. Both works explore the fundamental issue of what happens when a political order is expansive enough into the realm of personal choice. For both, the pursuit of a perfect political and social ideal is something that removes individual freedom in the name of a realm where "perfection" and social design becomes the most sacred notion of the good. I think that there could be a very strong exploration on how the community operates in this realm of preserving perfection. The idea of being able to construct a realm where suffering is not readily apparent, yet a domain where human happiness and exploration is absent is something that can be explored in both works to a great extent in the work of Lowry and Westerfield.