Adding Shrimp To A Community Aquarium

Because shrimps don't get the same diseases as fish you don't need to quarantine from that point of view. However, they will be in a tank at the shop which more than likely will also have fish in it or the tank will be on the same filtration system as tanks with fish in. There is the slight possibility that something from the shop could end up in your tank and get your fish eg if they're in a tank with fish that have whitespot, there is the theoretical possibility that some stage 2 parasites could get into the bag.
I would be inclined to quarantine them but only long enough for any bug to die out if it is unable to infect a fish - as long as there are no fish in quarantine with them.

There is one other reason to keep them in a QT and that is if they are very small. You might want to grow them on a bit. Adult amanos are quite big shrimp and should be safe from your fish, but tiny amanos might not be.

As for too many bottom dwellers, I found that amanos don't spend much time on the bottom. They seem to prefer climbing through plants and decor.

The 25 litre would be fine.
A week would be OK if they are big, or until you think they have grown big enough that you biggest fish won't be able to eat them if they are small.
They don't add much to the bioload so anything up to a dozen would be fine. They do like to be in groups so no less than half a dozen.
Do you have any algae pellets? I have never fed my amanos anything specific; I have one left from a batch bough in Feb 2009 so they were fine with that. They do eat fish food, if you have any algae rich food, grind it up and feed just a bit.

They would be perfect. Don't give them too much at once! The reason to grind them up is that the first shrimp to get to the food usually runs off with it. More, smaller bits mean they all get some