Hi John,
Welcome and good luck. Nice reef BTW.
Plants for the cichlid tank could include Java Fern and Java moss. Bolbitis and Anubias species may also work well. Vallisneria and Sagittaria species are both grasslike and can be short or tall varieties and would do well in cichlid water (high ph and k/g). All are tough but attractive and all like to attach to rock and wood (not the sag and val though). Fish don't like to eat them for the most part and they provide shelter/breeding sites for all fish.
I think the filter idea will work well, but how will you cover the bottom but keep these inlet/outlet clear? I can see the outflow back into the tank spewing substrate all over based on what you use. Plus, how will you keep the bottom strainer clean? Will this be raised and covered with a sponge and hidden???? How will the outflow be directed?
The height of the tank bothers me for several reasons, but can be countered:
1. Surface area for the exchange of gasses at the water surface. This can be enhanced by more/better flow throughout the tank. Really depends on your final bio-load and filter size/type.
2. Cleaning the inside of algae or just aqauscaping in general. I hope you have long arms, because it will be tough to stand on something AND reach in to the tank.
Good luck with that
3. A deep tank is more difficult to light if it has plants. This may take you into MH territory, which you should have from your reef days. Switch to 6500k bulbs......
I would also try and select fish that like to do laps for this 100g tank. Angels or Angels would work well in this setup IMO.
Hope this helps.