Is the idea that there would be predesignated battles with a known place and time and number of troops on each side for giving the army experience, and gaining captives, a real thing?
Additionally, did people like say, the Tarascans do it? OR was it a distinct Aztec/Tlaxcala diplomatic relation.
As far as I can tell they did indeed happen. I know of one article on the subject (Hicks 1979). This author suggests that purpose of Flower Wars was not to obtain sacrificial victims, but merely for military training or practice. Although this article is few decades old so the current academic consensus may be different.
Article abstract:
"The notion that the Aztec fought "flowery wars" primarily to provide captives needed for human sacrifice is called into question. The data on wars called "flowery" in the ethnohistorical sources indicate that they were simply wars not aimed at conquest, and that the most common motive for waging them was to provide military training and practice. "
EDIT: another article on the subject
"This article examines the sixteenth-century Hispanic writings on the so-called Flowery War between the Aztec Empire and the Tlaxcala-Pueblan Valley kingdoms (ca. 1455-1519). It argues that modern writers have grossly exaggerated the ritual component and intent of this protracted war, largely through a misunderstanding of the geopolitics of both the Valley of Mexico and the Tlaxcala-Pueblan Valley. The thesis presented here is that the Aztec Empire was simply unable to conquer the Tlaxcala-Pueblan Valley, despite high incentive and heroic attempts to do so. "
/r/askanthropology may be able to help