@pseudoprimal hard to compare math levels, though I will say that kids who went to HCSSiM one summer sometimes go to Ross the next summer, but the vice versa isn’t really true.
Most people at Ross have completed Calculus. In my group of friends, the first-years who were rising juniors had mostly all completed a couple of semesters of math past Calculus, but I can assure you that this isn’t necessary. The specific material you learn in Calculus (or Linear Algebra, or whatnot) won’t really be of help to you* – again, it’s all about mathematical maturity. The kids at Ross who were scary-accelerated were looking for a challenge in previous years and have torn through a lot of material in a short amount of time. I definitely learned how to be average there.
Another note: a bunch of people at Ross were rejected the first year they applied but accepted the next year, so there’s really no harm in trying! If you can figure out the problems on the Ross application and are 15 or older, and you feel like you would like to spend all of your time doing proofs without guidance from adults for six weeks, apply. There are some really nice threads about Ross on here, including http://talk.collegeconfidential.com/summer-programs/867050-why-you-should-consider-the-ross-mathematics-a-letter-program.html .
*except for if you take Abstract Algebra or another class that actually does fill in the foundations of Number Theory. This is one reason why kids who went to another program the previous year tend to be equipped fairly well to deal with the math; a lot of math programs teach Abstract Algebra.