assuming you do not have a bunch of Youtubers in your network :) and you only want to shape streams from from Youtube (not uploading of videos), then you will need to create a shaping policy for Youtube application with reversed 100Mbps shaper and 2Mbps per-ip shaper .
Example of shaping policy that should work for your needs:
config firewall shaping-policy
set name "shape_youtube"
set service "ALL"
set application 31077 # Youtube app ID
set dstintf <your_WAN_port> # Interface(s) towards your ISP(s)
set traffic-shaper-reverse <shaper_name> # Your 100Mbps shared shaper
set per-ip-shaper <shaper_name> # Your 2Mbps per-ip shaper
set srcaddr "all"
set dstaddr "all"
Note: For application-based shaping policy to work correctly, ensure your firewall policy has application control enabled.