I don't think there need to be other religious symbols in the Polish Sejm, because there is no past behind it. The cross in the same, has a past, so leave it be.
It doesn't have a past. It was put there in 1997 under the cover of darkness by people who had no business being there.
A cross on the wall in the Sejm, a school or work place, does not mean, if you are there, then you believe in it, and adhere to it.
Who asked if the people working there and studying there wanted a cross? Shouldn't it be up to them?
Does it really impede their life?
Perhaps you could start by reading the Constitution. I know you haven't read it (despite your patriotic claims) - but really, a quick look there will tell you all you need to know.