Might I suggest pygmy or hastatus corydoras?
In my experience they are more mid dwelling than others...
For another alternative, but more bottom dwelling, I am drawn to aspidoras over corydoras... They are thinner and shorter and more active, but still closely related until potentially reclassified.
Personally I have a 30g tank with unidentified tetras, corydoras hastatus, otocinclus, red rili and tangerine tiger shrimp and am more than happy with it.. (The light may be another story..)
I guess I can join the bandwagon and offer suggestions for the layout too.
Firstly, it's art, and it's yours.... ultimately you will be the one looking at it, and enjoying it... so it has to appeal to you primarily..
The rules are there more as guidelines to try to influence how you will look at it.. not necessarily a set of hard and fast laws that can never be followed.
But, the rocks (Based on their appearance to me) or the wood (Based on you saying tannins) don't work for me.. they aren't big enough to dominate, I like their look, but I worry that when you get plants, they will be dwarfed by most plants, so you won't get to see them.. again, my preference is for larger plants... if you prefer a lot of little and short varieties then you'll be using less than half the tank, something taller and you won't see the hardscape.nOt using too many plants, will make this less of an issue
Also, depending on the sort of plants you want, it looks like you could use a little deeper substrate too.
But... I think the best thing to do, is keep going forward, and make changes if you want.. it's going to be something you can always change around if you want