when we create ethereum HD wallet through a mnemonic phrase, we always need derivation path. is it fix for a particular currency or we can put whatever we like (ex- m/0/n, m/4/3/n or anything else)?
If your wallet allows it yes.
There are some wallet that only use a predefined path, so if you try to import a seed they will not show any ether/tokens that are in another path.
If you use other path you should be aware that changing path might alter your security considerations, ie consider hardened keys.
Also some of the places in the path derivation have explicit meaning like coinType and there's a register for common coins.