I have a minor in the subject I want to get a master's in (CS). Core courses, all excellent grades. I have a major in math. If I get all 3 of my recommendation letters from math people (or possibly 2 solid math letters and 1 stellar letter in a totally unrelated subject), will that look bad to the CS admissions committee? As core courses, the CS classes were all massive lecture halls and I didn't develop personal relationships with the profs.
Oh, it doesn't really matter.
If it is course-based CS master, not researched-based CS master, then a recommendation letter only takes a very very small portion of it, so whether it is from a math professor or a CS professor doesn't differ too much.
GPA is the most important.
One of my friends who worked in the admission process before told me that the first admission step is to filter out students whose GPA is lower than 3.5 no matter which university they were in.