The hole pattern on the Vortex Venom is the same as the fastfire. The razor is not the same. I don't remember about the RMR, sorry. But, I think all of them can provide adapter plates at a cost.
The fastfire is not at the same price point as the others and is more in line with the vortex Venom. Honestly, if I were to look at a new optic and didn't care about money I would go with a new Aimpoint ACRO.
Otherwise, I would get a Venom since it will fit your mount. The Venom has more MOA adjustment than the fastfire and 10 intensity settings, both nice features. Lifetime no bs warranty is good as well. Good luck.
