I'm afraid to take a nuclear cardiac stress test because it sounds radioactive. Is it dangerous?

Nope. There is radioactivity involved, but the dose is not considered in the range that it presents a risk and is comparable with other radiology exams. If a physician is ordering the exam, there is typically a good reason that would present more of a "risk" than the nuclear scan. I would not worry about the word "nuclear" in the test and take care of your heart.
No. It uses a very low dose of a radioactive chemical that dissipates quickly after administration. It's not much different than a ct scan.

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Will cardiac stress test, and heart ultra sound show if I had atherosclerosis? Does homocysteine play any part with it?

Atherosclerosis. If the ultrasound shows evidence of vascular calcification, that is diagnostic of atherosclerosis. A positive stress test suggests the presence of atherosclerosis. Homocysteine is only loosely connected to atherosclerosis. If you are worried, become a vegetarian. Read more...