We recommend that you install it on its own domain, even if it is on the same server as some of your other sites. FunnelFlux will also work with subdomains.
Just be sure not to combine FunnelFlux with other apps/sites within the same (sub)domain.
E.g. installing FunnelFlux on mydomain.com then also hosting another app in mydomain.com/my-app/ is not advised.
The reason we suggest a dedicate domain is that, if in the future your traffic increases exponentially and you want to get a new and faster server to handle the load, it will be straightforward to do.
You will just need to export the database and files to the new server and change the DNS records for your domain to point to that new server.
Doing this would allow you to experience no downtime during the switch.