I will take 6 international alevels edexcel

I have two questions:
1) Can I take exams in both the January and June sessions?
2) Will Cambridge/imperial really care if I take 6 alevels as opposed to 4? I know for Math I would take STEP, however I need to be admitted into the interviews first, and I would like to apply during my gap year(having already gotten my results for the 6 international alevels).

I want to study either math or computer science.
My subjects are:
Maths and Further Maths
Physics
Computer Science
Chemistry (may drop, not too sure)
Economics