JOBO do a sheet film loader for the 2509 reels, which is handy, but not really necessary. If you get a 2509n reel with the black clip-in diffusers, you should be able to do six sheets on a reel. If you get a 2509n without the diffuser (or a 2509 reel which is the earlier version), JOBO recommend only doing four sheets at a time for consistent results and preventing streaky development.
For the 5x7s. You can use the paper clips to keep them in place, though I have had no problems without the clips. I had one come loose once when processing film, marking the emulsion slightly, so I have tended to go without the clips. Though I do still use them for half-plate, the smaller film does not sit as firmly against the ribs of the drum.