Personally I would want to give the new Doe a week to settle in before even attempting any introductions. 8 weeks is a very delicate stage for a Kit, especially regarding their GI tract. The balance of gut bacteria is changing and any stress can disrupt this causing gut problems such as diarrhoea. Being moved to a new home is stressful enough, bonding straight away is adding to that stress. Also, it is a good idea to quarantine any new intake for at least a week to make sure that they are not incubating any disease. Again this especially applies to baby Rabbits.
These are just my personal opinions, I am not saying that it is what
has to be done, so please dont read my post as being critical of your actions.
As the older Rabbits are 20 weeks then there is a strong risk of an unwanted (I assume) pregnancy. A buck can retract his testicles back into the abdominal cavity so it may be that he is fertile. It is not impossible for a 20 week old Doe to conceive. It is of course entirely up to you if you want to keep them all together, I am just flagging up the risks in case you are not aware of them.
In the videos the older Doe is showing some aggression- tail up, circling etc-. She is also trying to get the Buck to acknowledge her place in the group since the arrival of the new Kit- putting her head down to him. He did not respond (by grooming her head). If this type of interaction were to continue the older Doe is likely to get cross and friction could set in between her and the Buck. The fact that the older Doe is about to hit puberty will just add fuel to the fire.
So in short personally I would get all the neuterings sorted out before attempting any bondings. The Buck can remain fertile for at least 4 weeks AFTER castration, I always leave 6 weeks before any introductions. The post castration fertility issue is noted on the following link were it says:
Note: following castration, the buck may remain fertile for at least four weeks and sometimes longer than this; the buck should be kept apart from any does until six weeks after castration.
http://wildpro.twycrosszoo.org/S/00...s/Indiv_TechniquesRabbit/CastrationRabbit.htm