One thing that you may wish to take into consideration is the fact that The Rivals is a farce, and in farces characters are often stock characters. That is, they are fictional characters based upon a stereotype. For instance, Lydia represents the incorrigible romantic. Therefore as a type in a farce, one may wonder how much of a social view is being expressed. It may be that the playwright is just using this type for comedic means.
In the previous response on the first question, there is a citation from the Epilogue of The Rivals:
Man's social happiness all rests on us:
Through all the drama—whether damn'd or not—
Love gilds the scene, and women guide the plot.
From every rank obedience is our due—
Perhaps, then, the third line that is in boldface may guide the thesis for this paper: With both the idealistic and the realistic, women direct the social happiness of men in three comedies,....(list them)in which (1).....(2).....and (3)......(you need at least three results that occur with the men).
The student can, then, discuss the effects of each type of female character upon the lives of the men with whom she is involved.
The outline for the paper could follow a comparison/contrast among the plays, demonstrating in which way the idealist affects outcome and how the realistic character does so, too. [Sorry that this is sketchy, but without knowing what is going to be written, it is difficult to be specific]
***For your convenience there are 2 links that should greatly help with the composition of the outline.