Is it wrong to make assumptions if you're a scientist

Is it wrong to make assumptions -- scientist or not, its only wrong if you stubbornly cling to them in the face of contradictory evidence.

as far as Einstein, light being the cosmological speed limit wasnt an "assumption he made"... it was the inevitable conclusion he was left with after all the facts, evidence, and text conclusions had been assimilated and understood. The people that believe it based on solely Einstein's credibility are the ones I feel sorry for. Einsteins conclusion is based on the work of HUNDREDS of other scientists, some of whom spent a lifetime dealing with primitive equipment laboring under an oppressive and intimidating government and Church. As far as Newtons work, his conclusions are regarded as Laws. If they aren't correct, then the simplest most basic fundamental beliefs and understandings we have of the universe are wrong. They are a "foundation" not an assumption. Most other scientific conclusion are based on the inescapable infallibility of Newtons work.

You are DEFINITELY too skeptical. Challenging Newton is bordering on the Metaphysical, your asking how we can trust our own senses and our own ability to perceive and measure. With out being able to accept the evidence of our own senses, we may as well just resort to philosophy and the "nothing can be truly known" mentality of Descartes.

SO, again, MAKING assumptions is vital, clinging to them is foolish. Science is based on facts and evidence, the result of experimentation, subject to peer review for approval.

You have to assume something. But that isn't science.
It becomes science when you test you assumptions.
I am sure you know the scientific method. Make a theory, do experiments to test it, and if it fails the test make up a new theory and a new experiment and test it again.