Q.What documents are required to carry IMDG cargo Class 6.2 and 7? Is any certificate required and from whom?

Normally for transporting IMDG cargos, ships must have Document of Complaince which will be issued by Flag State.

https://navsregs.wordpress.com/2017/01/24/document-of-compliance-for-ships-carrying-dangerous-goods/

But for Class  6 and 7 dont need DOC.

Since IMDG class 6.2 is Infectious subtance you must refer the

Guidance on regulations for the transport of infectious substances 2017–2018

Applicable as from 1 January 2017 published by World Health Organisation.

https://www.who.int/ihr/publications/WHO-WHE-CPI-2017.8/en/

For IMDG Class 7 - Radioactive Materials include uranium and plutonium.

INF CODE ( International Code for the Safe Carriage of Packaged Irradiated Nuclear Fuel, Plutonium and High-Level Radioactive Waste on Board Ships)

https://www.wnti.co.uk/media/3762/5.pdf

Survey and certification

A ship that is certified for the carriage of INF cargoes is subject to inspections and surveys, as required in SOLAS74, chapter 1. Prior to transporting INF cargoes the ship must be internally surveyed, including a complete examination of its structure, equipment, fittings,arrangements and material. On passing an initial survey,an International Certificate of Fitness for the Carriage of INF Cargo is issued. This Certificate ceases to be valid if surveys have not been carried out or if the ship nolonger complies with this Code when the Certificate has expired.