Yes, you can enable it globally using "spanning tree portfast default"
Your switchport has to be access mode in order to take effect
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Yes, it won't impact current port activity.
BTW, if you use portfast, you should also use BPDUGuard. Even with BPDUGuard, you increase the chances off a L2 loop happening, but this additional risk is often considered acceptable. Lastly, are you using rapid-STP? If not, you might want to consider using it.
You can enable this feature if your switch is running PVST+, rapid PVST+, or MSTP.
Beginning in privileged EXEC mode, follow these steps to enable Port Fast. This procedure is optional.
Enter global configuration mode.
Specify an interface to configure, and enter interface configuration mode.
spanning-tree portfast [ trunk ]
Enable Port Fast on an access port connected to a single workstation or server. By specifying the trunk keyword, you can enable Port Fast on a trunk port.
Note To enable Port Fast on trunk ports, you must use the spanning-tree portfast trunk interface configuration command. The spanning-tree portfast command will not work on trunk ports.
Caution Make sure that there are no loops in the network between the trunk port and the workstation or server before you enable Port Fast on a trunk port.
By default, Port Fast is disabled on all interfaces.
Return to privileged EXEC mode.
show spanning-tree interface interface-id portfast
Verify your entries.
opy running-config startup-config
(Optional) Save your entries in the configuration file.