What about using a switch ? Use one of the Juniper LAN port, put a 3 port switch (if it exists) on it, use port 2 f the switch to the existing network segment and the third to the pfsense box.
Of course, bandwidth goes up to the capacity of the LAN port of the Juniper firewall.
The DHCP server of pfSense isn't really from "pfSEnse". Its a industrial strengh DHCP server that is already known and being used on FreeBSD for years (also a reference). It can handle a lot of DHCP request … the only limit might be network bandwidth and "pure processor power" of your firewall. This is valid for your actual Juniper firewall, or the pfense box. I guess all possible setup-possibilities are there.
I'm using pfSense as the main "firewall" at my work - about 10 PC's and seperate a Portal Wifi subnet for our customers (a hotel).
I use a PowerEdge from Dell to handle it all (or an older Dell Dimension 51xx, an old retired desktop PC) with a quad NIC Intel PCI card - the onboard NIC is my WAN NIC. It runs fine for years now (it only breaks when I mess up the script/code ones more).
Here are the stats: https://www.test-domaine.fr/munin/dyndns.org/brithotelfumel.dyndns.org/index.html
As you can see, my 'firewall' is just twisting its fingers all the time .....
You could also consider buying a special appliance as said here: https://www.pfsense.org/hardware/index.html#sizing - stuff like this (example) http://store.netgate.com/Netgate-FW-7541-1U-Rack-Mount-System-BTO-P1903.aspx (6 Giga NIC's) will handle hundreds of PC's easily.