I know that to enter any level 2 program, i need to have 30 credits, but with those credits, do they all have to be 1st year courses, or can they be 2nd year courses provided i have the prerequisites?

I'm 95% sure that second year courses would still be allowed. Also, I don't know about your faculty/level 2 program choices/whatever might make it different, but you don't need a full 30 units to get to Level 2. I dropped a course second semester of last year and so only finished with 27 units, but I still got in to Level 2 for this year just fine. I'll just have to make up those extra 3 units some time before graduation, so I'm overloading next semester.

I only had 24 first year credits when I got into a second year program in social science at least. The other 6 were 2nd year.

No they don't have to all be first year courses. All you need to apply for a second year program are the course requirements.

For example, if you wanted to be in Honours PNB, the requirements are as follows:

Enrolment in this program is limited and possession of the published minimum requirements does not guarantee admission. Selection is based on academic achievement but requires, as a minimum, completion of any Level I program with a Cumulative Average of at least 6.0 including:

6 units PSYCH 1X03 (or 1AA3), 1XX3 (or 1A03) with a grade of at least B- in each
3 units from MATH 1A03, 1LS3
6 units BIOLOGY 1A03, 1M03
3 units from CHEM 1A03, PHYSICS 1B03, 1L03 (See Admission Note 1 above.)
6 units from Life Sciences I Course List (See Admission Notes 1 and 2 above.)

(taken straight from the Undergraduate Calendar http://registrar.mcmaster.ca /CALEN...nt/pg1631.html)
You should get familiar with it.

On an unrelated note, you can't be in first year PNB, as we don't have a first year PNB program.

They don't have to be all first year courses.