Hello dear student, 

You can attend th open counseling round if you want to because you cannot be sure if you will get a seat there or not . While as you said you have already joined a medical college so it's really good . But try there is a probability that you get a seat in Aiims and if the college you get is suitable for you then further you can see. If it will be suitable for you to join it or not . I hope this was helpful and yes you are eligible to join open counseling for aiims 

Best wishes