Juniper EX4500 selective q-in-q This switch supports selective qinq? For example I got 3 vlans on client port: 100, 200, 300. I need to add second tag on vlans 100, 200 but vlan 300 must run as is, without adding second tag. Is it possible?

Yes it is supported - please see the answer here (for EX4200, but same behaviour can be expected on EX4550):

Juniper EX4200 QinQ question