BenPen, as I wrote before - not fish produce waste, but you do. It's same with humans and all the animals - if you won't feed them, they won't produce waste. When you feed them - part of the food goes to growth, reproduction, energy and remaining comes as waste (in case there're any leftovers, then leftovers = pollution).
As plant eating fish - I doubt that you'll be able to tell them - eat this plant and not the other...
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Fast growing plants lower the no3, so the water changes generally. Good filtration can also help a little with no3 in case of anoxic environment. But I doubt that you'll be able to do that with sump. Also, I don't see a point in using all the other filters in your situation. What do you try to achieve? more surface? Bacteria grows on all surfaces - inside your aquarium as well and the largest surface is your gravel/soil. The sump will be enough for bio and mech. Having a "prefilter" sponge on your inlet pipe would help for the sump to keep larger particles outside of it and serve as a mech. filtration too.
Carpeting plants. I can't talk about the "hard ones" like Hemianthus callitrichoides, which probably require co2 in order to survive..., however I never used co2 (the reason behind that - I don't want to trim it often and the usual issues with algae comes much faster... basically all the problems in tank needs much quicker reaction). Just do a dry start and plants like monte carlo will be ok (just not ok with large bichir…)
Rainbowfish will probably become food for larger fishes, but if you have other tanks or If you get them fully grown, they would be good for quite some time. When you see them starting to disappear - rehome to other tank or just let the nature take its course. (I've seen amano shrimp living with Erpetoichthys calabaricus and red cherry with male betta. Both somehow survive to this day (6-10 months) - after that, all I can say - It really comes to character of the fish)
I'm far from expert, but I don't see a point in thinking "will my fish be ok when fully grown...". Unless you can get them already grown up, you'll face many difficulties along the road to fish maturity (like your oscar story...) which might naturally show that only the "strongest" (or luckiest) will survive. About the water changes - I also can contradict the opinion on weekly water changes. I always wondered about those with their "I always do weekly 50-75% water changes" and their vocation? Or their family goes on vocation and they stay home to do their water changes? Just get many tests and test water every 2-5 days... When stocking your aquarium - same. Introduce the fishes one/few at a time and check the readings. Wait for few weeks when constantly checking parameters. Bacteria will adapt to increased, your provided, food load. The only reason behind weekly water changes is that new water introduce elements needed for plants (and probably fishes), but there won't be a difference if you do it daily or weekly or almost monthly. Check parameters and if needed - change water.