network printing

Certainly... assuming customers can get to the DMZ web app, you'll just need to allow your DMZ IP to your LAN printer IP over whatever protocols you need for your print server. 

Wireless is irrelevant, unless wireless traffic doesn't go through the Firewall. 

​@tonyferguson why smb for printing? With most printers you just need IPP (and maybe SNMP for getting Toner values)…

As long as one doesn’t want to use Airprint hopefully...

It can work, if the printers can multicast through the FortiGate and the clients can reach the printserver and the printserver can reach the printer.

Yes, this can work, but I’d avoid opening broad DMZ-to-LAN access. A couple of safer options:

1. Let the LAN pull the orders. Run a small print agent or service on the LAN that checks the web app over HTTPS for new orders and prints them locally. That only needs an outbound LAN-to-DMZ rule, so nothing in the DMZ has to reach into the LAN.
2. If the DMZ must push to the printer, create a very specific policy: source = the web server only, destination = the print server (or printer) only, and only the one service you need, such as IPP (631) or raw printing on TCP 9100. I’d skip SMB/NetBIOS (137-139, 445) from a DMZ if you can, since those are much riskier to expose.
3. Some receipt printers can poll a URL on your server themselves (for example Star CloudPRNT or Epson Server Direct Print), which avoids any inbound rule to the LAN.

Wireless printing isn’t really needed here, and I’d keep the printer or print server in its own VLAN with logging turned on.