Is it okay to put a 2U server in a 2-post rack?

I don't think I'd be comfortable doing that either. We use Two post Rack Shelves to help support 2-4U server in 2 post racks.

If it was me I would not flush mount a server in a 2 post rack I would center mount all my servers. I don't like to put servers in a Relay rack as they are built more for telco and network equipment. They will work for servers as the mounding brackets have been manufactured to work with the Relay racks. But in general I don't like to use relay racks to mount servers.

If it were me, and the manual said that it is supported in a CEA-310-E compliant 2-post rack, then I would verify that my rack meets the compliance and call Dell to find out what mounting hardware to use.

If you can center mount it, it should be fine. I wouldn't flush mount it due to the amount of torque that will be put on the mounting hardware from the rear end of the server just dangling in space.