I like your stock list but I do thing cherries will get eaten by the gouramis or rainbows. You'd be better trying amano shrimp. A bit bigger (and actually much better at clearing algae). I would keep far more Otos in that big tank. 6 isn't enough. I have 10 in a 120 litre tank and you rarely see them out. If you have heavy planting I'd chuck in at least 20 (and they'll be far happier as they are nervy fish).

Remember that many planting substrates can lower pH. I have ADA substrates and they all lower the pH of my tap water by 0.5-1.0. I've heard they also soften it too but I've not tested this yet.