Using Small Business Server Standard 2011, what is the preferred best practice for a group of about 15 computers.
To use it as the router/firewall and have it be assigning DHCP, or have it connected with the rest of the network via a switch?
SBS 2003 used to ship with ISA Server which would allow you to use the SBS box as a firewall/gateway.
It is no longer supported to have SBS 2018 or SBS 2011 as an edge device. Thus your only option is to have the SBS sit on the network like a normal server would and have something else provide your internet connection.
The internet device can be "dumb" - in that you don't need to use its DHCP or WiFi, you can still (and should) have SBS serve your DHCP requests.