Impeachments are tried in the Senate rather than in the courts, so the usual rules of procedure don't apply. Very few people are experts in the Senate's trial rules, and I am not among them. I doubt anyone else who received your question is, either.

It would not make much sense to forbid the impeached Justice to testify, so the rules most likely allow it. The Senate may even be able to force him to testify, though he would be able to invoke his Fifth Amendment right against self-incrimination when and if a particular question creates that sort of problem.