a few notes. The term "grain size" applies just to silver halide material, as there are grains of silver halide (often abbreviated AgX) dispersed in the gelatin layer. Materials such as photopolymer do not have any grains of anything like that. This results to an important fact: grainy materials based on AgX tend to scatter light and limit achievable resolution. Both of it limit its usage in holography. Yes, there are AgX emulsions for holography that are very fine grain, but one has to look for one. AgX emulsions for Lipmann photography should be fine grain too, but there is no certainity they will be the best emulsion for holography.
I have very good experience with low power red laser diodes, say up to 10 mW. I was able to make a decent hologram with any of them I ever purchased. I don't have such experience with other colors or higher powers. I know some people succeeded with strong green laser pointers, but I guess they were just lucky they got a good piece and worked in suitable environment.
I would not say it is "trial and error", I'd call it "hit or miss" with high probability of "miss". The problem is that if you are lucky, you don't know about it. If you are not, you have no idea where is the problem -- it may be the laser itself, it may be the power supply, optical setup, weather (no kidding), the object, the photosensitive plate, the development process -- anything.
My recommendation is: If you want to play with a powerful RGR laser, order a few low power modules as well, such as this one:
https://www.laserlands.net/diode-laser- ... 1001a.html
You can start experimenting with the RGB module, but I recommend to start with the low power one. Unscrew the collimating lens (use pliers and a vice), power it from common batteries and make a simple reflection (Denisyuk) hologram of a coin in contact with the plate. If you don't succeed, you can try another laser module (that's why I recommend to get more than one). If you still don't succeed, ask for advice as this **should** work.
Well, and if you succeed, proceed slowly by replacing the components in the setup one by one. For example, you plan to use a microscope objective to widen the beam. Thus, you should use a low-power red module with the collimating lens attached (because you know that it works) and observe whether an objective ruins the setup, or not.
One more last thought. It is not very difficult to make your first simple hologram as a proof of concept. It will be most probably small, of a small object, monochromatic, and not very bright. However, making a high quality large RGB hologram -- that's a very different story. It is not very hard to learn how to ride a bicycle -- but qualifying for Tour de France is, well, a long process.
By the way, you have mentioned you want to make "reflection holograms from long distances away (10ft?) in rgb color". Could you make a sketch? I am not sure I understand. If you want to make a reflection hologram of an object located 10ft from the plate, you are out of luck even with the best equipment.