~/.ssh/config

Host backup
	Hostname bck.example.com
	User 43210
	IdentityFile ~/.ssh/id_rsa_bck_example_com

Host metrics
	Hostname 192.168.1.2
	User metric
        Port 12345
	IdentityFile ~/.ssh/id_rsa_metrics

$ ssh metrics
$ ssh backup

$ ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no me@example.com

as alias in .bashrc or config.fish etc..

alias ssh-notstrict="ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no"
# example
$ ssh-notstrict me@example.com

in .ssh/config

Host example
	Hostname random.example.com
	User alfred
        Port 2345
        UserKnownHostsFile /dev/null
        StrictHostKeyChecking no