Is it safe to take a dose of celecoxib for pain relief after a sclerotherapy for hemorrhoids?

Yes. Non-steroidal anti-inflammatory drugs, like celecoxib, and sitting on an ice pack can be very helpful.
Yes. There should be no interaction between Celecoxib and the sclerotherapy medication. Some physicians are concerned that use of anti inflammatory medications such as Celecoxib may decrease the effectiveness of the sclerotherapy treatments. I have not seen any decrease in results in my practice when patients have taken anti inflammatory medication.