states are prohibited by the US Constitution from making distinctions based on ones immigration status. Therefore, if someone is a resident of Texas, they are to be allowed to pay in state tuition.
Ok, lets break this down: states are prohibited by the US Constitution from making distinctions based on ones immigration status.
You have that correct.
Therefore, if someone is a resident of Texas, they are to be allowed to pay in state tuition.
You have this part wrong. Texas may not discriminate (that means charge a higher fee for tuition) against someone based on their immigration status. If you live in Texas, you're a Texan, regardless of what country you came from. Therefore, you pay in state tuition rates.
I cited the appropriate passage from Plyer v Doe twice to support this proposition. Is there something you didn't understand about the passage I cited?