Well with Kuhli loaches you want a minimum of 6 to really see their true beauty. Also I would be careful about putting larger fish in the tank, even at full grown it could fairly easily become a snack for something. These guys also like a sand bottom.. and with your appreciation of bottom feeders in general it seems that might not be a bad idea. As to more plecos, I dont see a reason why you couldn't add even a full size one (12-15") in there, but with all the others you might have to supply additional food such as blanched cucumbers.(dont forget to remove uneaten portions.) You could possibly also look into the various Corydoras. These like to school in groups of 6 but you can have 2 species with 3 of each.
Now as to the other fish you do have alot of choices. Cichlids in general have issues with planted tanks but there are certain ways to get it to work... most of the time. May want to avoid those. I enjoy my "characins" mostly the tetras, particularly black skirt or Black Widow Tetra
. These guys school and do best with at least 6, they tend to occupy the mid section of the tank. I like my danios as well, such as the Zebra Danio
or Pearl Danio
. You could possibly look into Glofish as well. They are a genetically modified version Zebra Danios. Make sure they are legal where you live. These are fairly active top dwellers and can be a bit aggresive to at first and with new additions but they usually calm down in a couple hours.. 2 days at most. Again.. school in groups of 6 or more but seem to do fine with other danio species.
As to cycling the tank fast... plants. Byron has written a 4 part article about plants(and a fair amount of profiles. You can search these near the top or click on the shaded names of fish and plants) over in the Aquarium plants section. You should go take a look at that.
As to your filter it depends on your fish. If you want to go with larger fish you will probably need a more powerful filter. Fish that come from rivers and are used to a good current will also require a strong waterflow. Smaller fish(The ones I suggested are rather small, only a couple of inches) will need to be able to avoid the current from the filter so they can stop and rest. Lake fish will generally prefer calm water and not appreciate the current a stronger filter puts out.
I kinda touched on the sand substrate earlier but I'll mention a bit more. Pool filter sand is fairly cheap and works well, but as with any sand it can be a little difficult to clean without stirring a whole bunch up. I believe sand, as with pretty much any non gravel substrate can potentially trap and build up toxic gases causing what I like to call the bermuda triangle effect. Bubbles build up and eventually burst out poisoning or even crushing anything on it's way to the surface. With your appreciation of bottom dwellers though this really shouldn't be an isue as they will aerate the substrate and not allow the gasses to build up.
Ok i'm getting tired so I gotta cut myself off there. Hopefully that gives you some ideas.
Welcome to the forums :)
Oh almost forgot the most important thing.. we need tap water levels. PH, ammonia, nitrite, nitrate.