I would get 12 pygmy cories. I doubt this species is much of a shrimp eater, given their size, but I've never combined shrimp with fish.
Earlier you said the GH was 6, is this 6 dH or 6 mg/l(=ppm)?
The wood and leaves can lower pH, depending upon the initial GH/KH/pH. It won't matter with all soft ater species, so let it do what it wants. Leaves will ad tannin to the water, staining it brown; not a problem, good for these fish, but not everyone like it. I used dried oak leaves in my pygmy cory tank as they werespawning all the time and this provides excellent first food (the infusoria as the leaves decompose). I let nature take its course, and at one point I had 30 pygmy cories in this 10g tank.
The only root tabs I willuse are Seachem's Flourish Tabs, and for swords these are incredible. You only use one every 3-4 months.