Q1: I did add all the youtube IPs in "/ip firewall address list" but do I have to add all those addresses in "/ip addresses"?
No. Not as far as I understand your problem description.
'/ip addresses ' is for IPs which are used for routing through or access to your router.
That includes 180.***.***.204/29 on your uplink interface and whatever RFC1918 space you are using to communicate with your customers / resellers.
Q2: Didn't understand "Same as for internet but with dst-address-list=youtube?" In which section do I have to use this?
It depends on how you limited Internet throughput. You said you had done it, but did not describe how. The youtube IPs are used for matching the destination address of traffic from your network to youtube, or traffic sourced from the youtube servers to your network, so that it can be treated differently from general Internet traffic. You did not tell us how your youtube traffic is delivered to you. For now, I am assuming it comes on the same connection as your Internet transit connection.
Q3: You are right on this. How do I separate or distribute both queues? By creating mark conn and mark packet?
That sounds probable based on how little we know about the specifics of how you have configured your internet rate limits.
Q4: You are right on this too but how do I configure on their mikrotik device, so that they can provide them to their clients?
Thanks for taking your time to reply.
Once your router is configured use its configuration as a template to configure 'their' routers. Just reduce the parent queues' limits appropriately.
When posts do not get replies it is often because the question is not clearly understood by others on the forum. Sometimes the question does not provide enough detail for more than a crazy guess. Your question had both issues. Diagrams and sanitized configuration samples from your device can help overcome language / jargon / vocabulary / conceptual barriers.