STP is disabled by default on any Extreme switch, so do not expect any problems. If you want/require loop protection on Extreme I recommend reading about ELRP.
Regarding the VLAN question: In ExtremeXOS, VLANs are not added to ports, instead ports are added to VLANs. The command above would enable the Extreme switch to "get" all of the VLANs on one single port, hence the keyword "tagged". If you want to configure what Cisco calls access ports it's a simple
configure vlan name-of-vlan add ports 3 untagged
This also works with port ranges like "1-4,8,10-11". Should you get an error message that a port is aleready assigned to a VLAN you can delete the old assignment via
configure vlan name-of-old-vlan delete ports 3
For more information, please refer to the Concepts Guide and/or Command Reference Guide for your specific ExtremeXOS release.