Pardons remove the individuals' ability to invoke their fifth amendment right
That includes protections against self-incriminating testimony.
IF subpoenaed to testify before Congress in the future, those with pardons would have to tell the whole truth or otherwise face the choice of contempt or perjury.
The interesting part that will likely be litigated is the concept of pardon acceptance. Those receiving pardons may face a consequential decision - whether to accept the pardon or not.
If a pardon is accepted, the recipient would not be able to plead "the fifth" because there would be no threat of incrimination.