I have just tested on my lab MX204 with a EX-SFP-1GE-T transceiver towards an EX2300 switchport and I will not expect this to work as what I see is as described below.
MX204 running 18.3R1-S1 and EX2300 running 18.3R1.
1G:
If I force speed 1g on the MX204 interface with EX-SFP-1GE-T transceivers (which is 10/100/1000 capable looking at https://apps.juniper.net/hct/model/?component=EX-SFP-1GE-T), I see link 1G on the EX2300 but not on the MX204.
RJ45 transceivers isn't supported looking at the HCT.
100M:
Forcing 100m in both end of the link result in link down in both ends.
If you need this 100 Mbps link, you have to inject an EX3400/EX4300 switch or eg. ACX1100 in between.
EX4300 will be able to runs as an extended linecard by utilizing "Fusion Provider Edge" on the MX204. This requires a license on the MX.