How easy is it to get into 2nd year courses in 3rd and 4th year and would it hurt me in any way as long as I get my third and fourth year requirements in?

Depends on the course, most of them reserve seats for 2nd years but it shouldn't be a problem. I got into 2nd year psych courses with no problem as a 3rd year.

And it's fine so long as you meet your program requirements for upper year courseload (eg. 60% of my courses had to be in 3rd/4th year).

What do you mean by easy? As long as you have the prerequisites for the course, you can take it. In terms of being able to get into the course, it would be easier since being in third or fourth year, your course registration date is before the second years would pick. I don't think it would hurt you in any way. This coming year I will be taking a second year course as an elective. There are many fourth year students who take first year courses as electives as well.