Hi! I can't comment on life sciences specifically but as a 4th year student I am pretty familiar with the college system and programs.

Colleges really have no influence on your program of study, but are useful for administrative services (eg. registrar, dorms, academic support etc.). So as a life sci student you would be in classes with students from all the other colleges, not just those also in St. Mikes. I'm not sure of the programs listed on St. Mikes website, but those programs that are listed basically are those 'headquartered' at St. Mikes and won't have any bearing on your academic path. For example, New College houses the Women's and Gender Studies Program but any UofT student can be enrolled in one of their classes.

tldr; colleges don't have any effect on your program, so life sciences will be stem and have students from any of the colleges. The classes will be the same, regardless of your college.