Hi Kamil! When I took ENG 1 last yr, you did not need to complete the 6-units of complementary electives to move to level 2. You do have to complete them at some time before graduation. I'm sure it'll be the same for this yr, but if you would like to be absolutely certain, you can just call the eng 1 office: (905) 525-9140 ext. 24646 and ask.
Anyways, you are more than welcome to take the 3-unit elective during the summer. It gets difficult taking them in 2nd yr and above, because you may or may not be allowed to overload that year (take more units than necessary). You have to, I believe, get persmission from the department you will eventually be in. This is what I was told by Sally Williams, when I had that exact same question first year
Long story short, better to take them either first yr or in the summer - that way you have nothing to worry about come 2nd yr. because it'll be over and done with.