Well the LFS isn't entirely wrong, as I mentioned, some fish will be more stressed out than others in solidarity. For instance, let's say you wanted a school of 20 neon tetras, from the standpoint of the fishes comfort, it would not be best to add 2 neons every 3 weeks until you get to 20. It would take far too long for one thing, but also the neons would be more stressed out than they would be if you introduced, say, 5 at a time every 2 weeks. Furthermore smaller fish generally have much lower bioloads and it doesn't cause as much of a flux in water parameters to add higher numbers.
So obviously how the individual fish are feeling in their new environmental transition is a factor, but another, arguably more important factor, is monitoring new inhabitants for things like flukes and worms which is really where the extended QT time frame comes into play. It's a lot harder to catch diseases early when the new fish aren't isolated and at the front of your attention.
Anyway, to your question on clean up crews. Bottom feeders and algae eaters both truly benefit from being added later rather than early on in your new tanks lifespan. I try to make myself wait until my tank has been running for at least 6 months before adding these groups of fish. It ensures that water parameters are stable and algae and other microfauna have had adequate time to saturate the environment. Supplementary foods are important but I find that the fish really benefit from being able to graze freely on established algae in the tank.
You can get by adding some algae eating varieties early and just feeding algae wafers, etc. but for other species, it's very important that they have natural food sources inside the tank already. This is the case for things like shrimp and otocinclus (which in my opinion are my two top choices for a 20 gallon aquarium. Your choices range from snails and shrimp, to dwarf species of plecos such as bristlenose and rubber lips, otocinclus catfish, and a few others including some live bearer species like Florida flag fish. There are others but I don't recommend them for you right now because of difficulty in keeping/size incompatibility.
I personally think based on your prerequisites that you would enjoy otocinclus catfish, they are small, active if given a comfortable environment, and semi-easy to take care of if you can manage to get them through the first couple weeks. Otos are infamous for dying early on but usually thrive if they make it through the transition from the wild to their permanent home. Just know that they are rather social and ideally are kept at a minimum of 5 fish and their success is largely dependent on natural algae in the aquarium. This may not be possible for you depending on the type and amount of schooling fish you end up picking.
My second choice would be amano shrimp, they're relatively cheap and easy and a good first pick as they are probably the most efficient shrimp when it comes to doing their job as a cleaner, they can survive off algae and algae wafers but will also polish off pretty much any other fish food if they can find it. I don't know how they would fare with the kribs however.
And if nothing else, a nice bristlenose is a very good staple for most people as well, maxing out at about 5 inches they won't get overly large, and are quite peaceful and easy to care for.
As for where these fish will spend most of their time in the tank, algae eaters are evolved for eating directly from surfaces and thus spend most of their time on the bottom/ sides of the tank. Though otos are particularly active and will often eat and rest on foliage and hard scapes as well as the sides of the tank. Though shrimp do swim around every so often and are really fun to watch.
I'll reaffirm before I go that for best results, it's ideal to wait for noticeable algae growth before adding algae eaters to a tank.
Hope this helps, good luck with your tank