Hey Jake, welcome to the forum!

I think the walleye would probably do better in the larger pond, but it's hard to tell for sure. Algae and moss growth has little if anything to do with good oxygen, so I wouldn't base the lack of them growing on that. I feel the larger pond would be a better candidate for the walleye as there is more room for biomass in it as well as deeper water refuge areas. Walleye don't like bright sunlight and will avoid it by taking cover in deeper water. Since you do not aerate your ponds, you will run into issues with a thermocline and a lack of DO below it. If you add the proper aeration system to your pond(s), this will open up the deeper water areas year round to your walleye as well as other fish and I think will be well worth your effort whether you decide to stock walleye or not. If you are not sure how to go about this, there are several members on here who sell and are very familiar with aeration systems. They can give you advise on the proper system, how to install it and start it up correctly.

Walleye rarely do overly well in ponds, they will survive but their growth rates are often very slow. Your idea of stocking the 9"-12" fish is well advised as small walleye are a favorite bass food because of their fusiform shape. Even at 12", a 20" bass could eat one. Going with the largest size you can will greatly increase their survival rates.

The fish farm advised you correctly that they would not reproduce(reproduction is possible, but EXTREMELY unlikely). Even if they did reproduce, the likelihood that any fry would survive to a harvestable size is even more remote.

If your sole goal is to add a bonus fish to your fishery, stocking walleye at 10-20 per acre is reasonable. Under average pond conditions, expect your walleye to put on around 2"-3" per year for the first couple years after you stock them and then their growth rate will drop dramatically to perhaps an inch per year there after topping out in the low 20" range with perhaps a large individual pushing 24". That would be a rare fish though... Expect the vast majority of your walleye to top out around 18"-20".

Do you have any forage species in your pond like golden shiners or threadfin shad? I ask this as walleye are not nearly as efficient as largemouth bass at feeding on sunfish like bluegill and redear. This means their growth rates will be less and they will not do as well if you don't have any shiners or shad for them to feed on.

Hopes this helps you out a bit... Keep posting, share some pictures and keep us updated on your progress!