Is it possible to find a triggers ID in the GUI or is it only via the API?
Open a trigger configuration and check "triggerid" in the URL.
Also does the ID refer to the trigger on a template level or host?
In the previous step, if you have opened a trigger at the host level, the trigger ID is for the host trigger. If you have opened a trigger at the template level, the trigger ID is for the template trigger.
Id can I just disable the trigger on a host and not the entire template?
Yes, you can. When viewing triggers for a host, just click "Enabled" link on the right of the trigger. Alternatively, select multiple triggers and choose "Enable selected" on the bottom left.