If you are not using tagged/trunk connections between the switches then VLAN numbers only have local signature so it does not matter what the VLAN numbers are. Now if you want to interconnect the 2 switches and have all interfaces in the same VLAN then I would suggest you change the VLAN ID on the EX3300 from 0 to 1.
In the original EX3300 code the default configuration use VLAN ID 0 for all the interfaces. In general VLAN ID of 0 should not have been used as I think VLAN ID of 0 should really be the bit bucket; not the case for Juniper EX. Anyway in later version of code the default value was changed to a VLAN ID of 1. In either case, using VLAN ID of 0 or 1, all the interfaces belong to same L2 [bridge] domain or broadcast domain.
If you change the VLAN ID on the EX3300 to 1, then you can interconnect the switches and all interfaces on both switches will be members of same VLAN.
Hopefully this helps you.