Could the waste heat normally expelled from nuclear plants run low temperature stirling engines or does it need a specific temperature threshold?
Yes, it could. There are Stirling engines where a temperature difference of just 1 C is enough to get them moving.
The problem with using low temperature waste heat is that the expense of capturing it (e.g. the cost of the Stirling engine) is comparatively high while the amount of energy available is comparatively low. Researchers working on waste heat utilization are pursuing applications where low temperature heat is available but other generation methods are impractical (e.g. power a health sensor on your skin using your body heat). They are also working on ways to improve the efficiency and reduce the cost of waste heat utilization mechanisms.