score:8
The slavery issue (at the time, 90% of Texans were neither slaves nor slaveholders) was addressed in the Joint Resolution for Annexing Texas to the United States, approved by Congress on 1 March 1845, which included a provision allowing Texas to be sub-divided into up to four more states with slavery being banned in states carved out of Texas territory north of the Missouri Compromise line and left up to popular sovereignty in states formed south of the line: Snopes:Partly true
The full article discusses the implications of the slavery problem in more detail and some of the other controversies surrounding Texas' admission.
There are a couple of points that I infer from my read of the full article.
There were multiple resolutions, most of which were rejected; that means that there was considerable "sausage making" involved in the resolution. Some language was the product of compromises. I'd go so far as to wager that there was more sausage making than logic.
Texas was larger than any other four states; even if that doesn't logically make a difference, it permitted classes of compromises that wouldn't have been possible with smaller states. I infer that they thought a state the size of TX deserved special treatment. (All my friends from TX will support that conclusion).
The slavery issue was addressed far more conclusively by the civil war than by the legislation.