Do 450 SM's need to be on firmware 184.108.40.206 in order to be compatible with the 450M AP?
Yes, I believe so.
From the 220.127.116.11 release notes:
When replacing a PMP 450 or 450i AP installation with a PMP 450m AP, the following upgrade procedure is recommended:
1. Upgrade the 450/450i AP to 18.104.22.168
2. Upgrade the 450/450i SMs to 22.214.171.124
3. Upgrade the 450m AP to 126.96.36.199
4. Switch over to PMP 450m cnMedusa AP
I had some straggling 450 SMs on 14.1.2 connect to our 450M APs today so I know it works without updating. I did update them to 188.8.131.52 right away though once they were connected.
Yes and No
They will register to a 450M AP in 14.1.2 or 14.2, but they will NOT utilize MU-MIMO beam forming meaning it will run slower than a regular 450i AP
You need to be running 184.108.40.206 to take advantage of the grouping when you go up to 200mbps