According to release notes of Mellanox OFED for FreeBSD support ConnectX-3 or ConnectX-4 devices and there is no support for ConnectX-2.
http://www.mellanox.com/related-docs/prod_software/Mellanox_FreeBSD_Release_Notes_v2.1.6.pdf
http://www.mellanox.com/page/products_dyn?product_family=193&mtag=freebsd_driver
Regarding the question if clients will be able to see each other, it more related to your network configuration like enable forwarding, use proxy arp, configure routing tables, etc. There are few different ways to do it and it there should be information on internet how to do it. Assuming that you'll run TCP/IP, the instructions should be generic and work with any other vendor.