Is it ok to take a flight after a nuclear exercise test (thalium test)? are there any side effects or precautions?

Yes but: It's ok, particularly if you weren't noted to have any concerning heart disease on the imaging study. But if you are flying within 3 days of the test, it's possible detectors at the airport may register your "radioactivity" causing issues with you going through security/boarding. Most radiology departments will fill out a form documenting the test and the reason why you may be "+".