Low-light tanks rarely need to be treated any differently than fish only tanks. As long as the flow is kept up (to prevent stagnant areas) the needs of the plants should be more than taken care of by the feedings.
Mixing substrates is normally not advised since it will look mottled very quickly (denser/larger particles will sink to the bottom). Also in low light environments the plants will be just fine with water column feeding (you shouldn't have any swords or other heavy root feeders in a low-light tank IMO
I've never used an undergravel heater, and have no problems growing plants fine at both low and high lighting. I'd be concerned about anaerobic issues with localized elevated temp areas (near the heater) where you will speed up the metabolism and thus generation of toxic gases such as hydrogen sulfide. Again in a low-light tank, its just not needed.
Keep up on the water changes, get a good algae eating crew if you can house them with your types of fish (I recommend Oto's, BN pleco, MTS, true SAE
, nerite snails, etc.), and you should be fine.