Yes, the Leapers UTG Super Slim Picatinny RMR Mount is compatible with the Holosun 407C.
No. This Leapers UTG Super Slim Picatinny RMR Mount is designed to fit RMR red dots and other optics using the RMR footprint.
The 507C X2 has the RMR mounting pattern, so I believe it will.
No, the Romeo 1 uses a unique footprint. This mount will not fit it.
No, the K models of the Holosun optics use the Shield standard footprint. This is the mount that would be compatible with that optic.
https://www.opticsplanet.com/leu-177154-d-point-pro-ar-mount.html
No, this is specifically for red dots that follow the Trijicon RMR base pattern.
Unfortunately that sight is not compatible with the Trijicon RMR mounting pattern. You would need a weaver/picatinny riser instead such as this: https://www.opticsplanet.com/leapers-utg-super-slim-picatinny-riser-mounts.html?_iv_additional-features=3-slots
No, the RMR has a different mounting pattern. However, the Vortex Venom is compatible with Burris Fastfire or Docter red dot mounts.