You can create a major that exists elsewhere if it doesn’t exist at your school. For example, a classmate of mine created an urban studies major at my college. Urban studies is a major at several other universities but it wasn’t at mine, so she created it. An individual major doesn’t have to be completely new (and in fact, you are more likely to get support for it if it is NOT completely new), so it’s totally fine that there are American studies majors elsewhere as long as there is not one at Rice.
However, usually when you create an individualized major, you are creating the major for yourself - not with the assumption that students behind you will also pick the major and follow your guidelines. Thus, the three-concentration model is irrelevant for what you want to do. You want to create a major that forms a cohesive set of knowledge in a particular area, and demonstrates breadth and depth across that area, while also utilizing the resources you have at hand (aka classes already offered by Rice professors). Take, for example, the existing political science major. Students are required to develop breadth - they have to take at least one course in each of American politics, comparative politics, international relations, and theory and methods. But they also have to build a concentration in one of those four areas. In addition, they had to take classes in different modalities - two different seminars with different instructors and introductory statistics, which enables them to understand the scholarship done in the field.
So your individualized major needs to do that. Frankly, the major that you have designed here is much too far-ranging and doesn’t concentrate enough depth into a particular area. A major is usually around 12-15 classes. Even just taking one class in each of the areas you listed is already 10 classes; that doesn’t give you the chance to develop depth in a concentration, and you haven’t even really formed a foundation in any particular field or two.
Basically, you need to 1) create a foundation in a particular field or several closely interrelated fields, or in a specific type of area (like American studies), with some foundational courses (usually 3-5); 2) build an appreciation for the theory and methods of the field by requiring some classes in both (usually 2-3), and 3) build a concentration or depth in a particular part of the field by requiring electives within a concentration (usually 4-5). You can omit #3 completely and just allow any electives, but there still need to be constraints. For example, for an American studies major it might be any 300- or 400-level classes related to America, to be approved by two advisors. Or for an urban studies major it might be any 300- or 400-level classes in social science departments relevant to urban studies, to be chosen from a given list.
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Now, the second to last paragraph sounds more like something that would be accepted - it sounds like political economy, where you combine studies of political science, economics, and policy studies together. That has a lot of advantages. Number one, you have the resources at Rice - there are already departments of economics, political science, and policy studies there, plus sociology and business if you want to draw on those departments as well. There are political economy majors at other institutions: Berkeley has one, Williams College, Georgetown, for example. You can use them as a guide.
Williams, for example, requires micro and macro for PE majors, plus an introductory political science sequence; four courses specifically in political economy (which I think you could find substitutes for), and three elective courses, plus one comparative political economy or general public policy course, one American PE course, and one international PE course. Berkeley’s curriculum is very similar, except they also require proficiency in a foreign language. Georgetown’s is also similar, although they also emphasize theory and methods courses (they require econometrics, analytical tools for PE, and economic statistics).
A major is usually 36 semester hours, or about 12 courses.
A sample plan at Rice might look like this:
Foundational Courses in Economics and Political Science - 6 courses, 15 credits
ECON 201 Microeconomics I
ECON 301 Microeconomics II
ECON 303 Macroeconomics
POLI 209 Introduction to Constitutionalism and Modern Political Thought
POLI 211 International Relations
POLI 212 Introduction to Comprative Politics
Two courses in statistics or methods, one from each department - 2 courses, 6 credits
Choose two of
ECON 307 Probability and Statistics
ECON 309 Applied Econometrics
ECON 409 Econometrics
POLI 228 or POLI 338 Policy Analysis
POLI 395 Introduction to Statistics
*Four courses in a concentration area - 12 credits/i
You could choose any four from a list. You might make the rule that 2 must be 400-level courses. Some samples might be
POLI 337 Public Policy and Bureaucracy
POLI 355 Political Environment of Business
POLI 336 Politics of Regulation (there’s also a 436 which is an advanced version requiring the stats class)
POLI 435 Seminar on Money and Politics
POLI 463 Comparative Political Economy
POLI 476 International Political Economy
ECON 239 Business, Law, and Economics
ECON 284 Foundations of Public Sector Economics
ECON 355 Financial Markets
ECON 415 Labor Economics
ECON 436 Economics of Regulation
ECON 450 World Economic and Social Development
ECON 451 The Political Economy of Latin America
ECON 452 Religion, Ethics, and Economics
ECOn 455 Money and Financial Markets
ECON 460 International Development
ECON 461 Urban Economics
ECON 479 Economic Modeling and Public Policy
ECON 482 Social Welfare and Distributive Justice
ECON 484 Public Goods and Public Expenditure Theory
POST 300 Public Policy Planning, Management, and Advocacy
POST 352 International Economics