With Optiminer you need "zstratum://" prefix before the pool address. Like: zstratum://eu1-zcash.flypool.org:3333. If you mining securely: zstratum+tls://eu1-zcash.flypool.org:3443
No, you do not need ""zstratum://" prefix before the pool address". You only need it if you are connecting with "tls", as in your second example. Mine:
optiminer-zcash -d 0 -s nexus:9999 -u testie -p x -i 7 --watchdog-timeout 5 --watchdog-cmd ./watchdog-cmd