[Heater Needed?]

another factor is going to be the equipment that you are running on the tank. Lights, filters etc are going to generator some sort of heat to run. you might just have to experiment and see. Personally for my 20L in my office with a canister filter and a fluval3.0 the ambient temp is 74, but the tank runs around 76-78. Also might not be a bad idea to keep an adjustable heater in your tank and set it on the lower end incase the heat in your house goes out.

[Angelfish care]

if the end goal is to have 5 or 6 in a 55g, I would. that way you can quarantine and grow them out together. 

[Angelfish care]

If it was my setup I would either keep the angelfish solo as a centerpiece in the 29g or 55g with a school of tetras or maybe some platies or mollies (I'm a sucker for livebearers). If you want more angelfish I would keep 5-6 total in the 55g, I think you would do better with more to help disperse the aggression.

[Fluval CO2 kit...]

I don't think so, I was getting ready to buy a 95g set myself and did some research into them and found more often than not people were having the same issues that you are.  I ended up going with the DIY co2 route myself and havent had any issues (knock on wood)

[Easy planter + jungle val]

Looks good! I have been really wanting to setup a tank with the same stocking, Discus and Rummynose tetra 😍 out of curiosity what temp are you running your tank at? 

[Question about Dwarf Red Aquarium Lily]

this is straight from the coop blog "Once the bulb begins sprouting leaves, you can gently tuck it into the substrate so that only a third or half of the bulb is buried." 

https://www.aquariumcoop.com/blogs/aquarium/dwarf-aquarium-lily-care

[Mbuna buddies]

How big is your tank? When I kept African cichlids I kept bristlenose pleco with mine and didn't have any issues. If you want a bottom dweller you could probably also do a small group of synodontis petricola.