Erm...interesting question.

1) Why a few pieces in the tank ?
To what i see is that LFS operators do not want to simply take too much a risk to themself especially in investing into a fish that most people have a problem to like. Not everyone will buy this fish so that means that the stock will sit in the shop for a long time and this in turn will be seen as consuming the owners resources in terms of fish food, and tank space. The price of one specimen is quite high too and buying more than 10 pcs is like getting some other stocks combined together. LFS owners are also quite worried that peradventure the fish dies (sickness, underfed, fighting or luck), then there will be a huge lost to him. So when u try to balance this costs together and make it work, its a very scary thing to think about. If it succeeds, it will pay off well but if the lfs guy do not know how to handle, then he has to fast for the month.
You can also consider this fact that the lfs guy is testing the market to see the reception of the customers towards this fish.

2) Shrimp friendly ?
Well i cant guarantee for all specimens of plecos. Generally those smaller ones will not prey on shrimps unless its already dead. If u have moss on your wood and place it from the mid level or higher in the tank, the plecos will not bother them. There are meat eaters and vegie eating plecos and also those that swallow everything like the common pleco
Pseudacanthicus and Leporacanthicus genus are generally carnivorous in nature so ur shrimps are n ot actually safe if they venture too near this plecos. Ancistrinae, Ancistrus and Hypancistrus are quite ok with shrimps...will leave them alone...unless they are already dead.
I had a Leporacanthicus Galaxias L240 in a shrimp tank...all my shrimp still around and got frys too. The L240 basically eats those that are dead or leftover food that droped to the floor.

So before u buy any pleco to put into your tank, do find out what they require for food and furnitures. Wood is a must for food supplement.