I'am used to do raf'ol yadayn in prayer... but when I am praying behind an imam, can I do rafol yadayn when imam didn't?
or shouldn't I do that and need to follow the imam?
If I do so, will my salat be weak?
The issue of raf'ol yadayn is not of whether it is permissible but of whether it is more virtuous. All Imams agree that it is permissible.
Suppose Imam is following Hanafi fiqh and not doing raf'ol yadayn, while you are following Shafai fiqh and doing raf'ol yadayn. There is absolutely no problem in you doing raf'ol yadayn while Imam doesn't. Here is a fatwa by Darul-Uloom Deoband in this regard. The converse is also true. If the Imam is doing raf'ol yadayn and you are not (as per respective madhabs), then also there is no problem.
You can do that even if the imam doesn’t.