At what age it is safe to introduce a comforter (with dummies attached) into the cot ?

It is important that there is no risk to baby’s head/face being covered by anything soft or loose in the sleep environment.
Red Nose has the following general information about soft toys/comforters.
Keep soft toys out of the sleeping environment for babies under seven months of age because they may cover the nose and mouth and interfere with breathing.
Some babies over seven months of age may appreciate a small object such as a soft toy to provide comfort and connection (transitional object) during times of separation from their parent.
https://rednose.org.au/article/soft-toys-in-the-cot

Shall I use a blanket as a comforter at this age? (6-7 months)

Red Nose has recently updated our information on soft toys/comforters, since the question response you commented on.
https://rednose.org.au/article/soft-toys-and-comforters
As you will see from this article, Red Nose does not recommend the use of pillows, blankets or toys with long blanket attachments due to risk of head & face covering