Regarding clns mtu

I understand the below points,please confirm if my understanding is correct.

1. MTU > MSS

E.g if my MSS is 1460 bytes, then MTU should be at least 1500 bytes(20 bytes for IP header and 20 bytes for TCP header).

I also understand that since the upper layer (MSS) will be finally encapsulated inside lower layer(Ethernet MTU), therefore MTU has to be greater than MSS.

2.I undertstand that CLNS is equivalent to Networking Layer of OSI model that ISO came up with.
where does clns mtu operate–at which layer(i know it is not any of the osi layer as ISIS was developed before OSI layer)–so what would be the relation of clns mtu and mtu ? Should it be greater than/less than Ethernet mtu ?

3.In some config snippets, where isis is running, i see the below config

clns mtu 1500

Why do we need the above command ?What if i remove the above command ?

4. is the following config valid(just want to test/understand in my lab)

   mtu 1400

   clns mtu 1500

Here, i have made mtu smaller than clns mtu. This will fail ?