A 207 as a single filter would be better for a 20L.
You can keep the 107 and add another HoB or sponge filter at the other corner of the 107 output to ensure more flow and surface agitation. That would be what I'd do. Having two filters is also safer if one fails.
Try cutting down starting tomorrow. Feed once per day, then gradually, maybe once every 2 days. If the fishes are alive and well, continue doing so. You get to save some food and probably also save some time for maintainance too.
Two of the most common shrimp kept in community tanks are cherry shrimp and amano shrimp. Cherries are more colourful but amanos are more prolific algae eaters. If you go with cherries, any centrepiece largish fish is gonna try to prey on them or their shrimplets. But your tank is well planted...
Stick with live plants. Even if you're new, go with easy low demand plants. They look much better and helps with water quality and ecosystem.
For stocking here is an idea
6 peppered cories
15 green neon tetras
3 honey gouramis
Green neon tetras
The above are some other options, with my favourite being the espei rasboras.
It's nice to have some bottom dwellers, so I'd still get the ASC. Otherwise 8 to 10 corydoras habrosus would be a great choice too
The honey gourami is a good shout, I have one in my 20g.
Another suggestion is a apistogramma borelli. They are ok with the temperature U have and more of a bottom half dweller, which might work better since you already have some tetras and Fabio's occupying the mid and top.
I have small tanks and I use more of the powder aquasoil as the granules are much smaller and wouldn't look much better in a small tank. For my 20g, i used the regular sized aquasoil at the bottom, and cap it off with the powder aquasoil.
The powder aquasoil is also much easier to plant into...
Excel is more a algaecide instead of co2 replacement for plants really.
If you want to run co2, might as well bite the bullet and invest in a proper co2 setup. Otherwise, a tank with low demand plants can also loon nice and lush.