I think I would use in when talking about a broad field and at when talking about a specific skill.

I have always tried to excel in courses relevant to veterinary sciences

 

I have always excelled at diagnosing cancer in small animals

 

I excel in laboratory environments

 

I excel at using the mass spectrometer

I'm not sure if its wrong to use at/in interchangeably but using them like this ^ is what comes naturally to me as a Brit.

I think I would use in when talking about a broad field and at when talking about a specific skill.

I have always tried to excel in courses relevant to veterinary sciences

I have always excelled at diagnosing cancer in small animals

I excel in laboratory environments

I excel at using the mass spectrometer

I'm not sure if its wrong to use at/in interchangeably but using them like this ^ is what comes naturally to me as a Brit.