The simple answer whether a single 20W T12 will be sufficient is no. You may be able to get by with low light plants but adding more is better. If you decide to only use one bulb then you should replace what you have with one in the 6500K range. I'm using 3 Philips Daylight Deluxe, Color temp. is 6500K, 1950 lumens, CRI of 79I and picked them up at home depot for about $5. I also use one in the 4500 range that I got there as well.
Lighting can be confusing to say the least. The red plants you have prefer more towards the 4500 range where the grass needs more of the 6500 (and a lot of it). When discussing lighting one needs to understand PAR (“photosynthetically active radiation”). Here
is a sticky that explains it pretty well.
I understand you impulse bought a few things but if you want your plants to do well a couple of things do need to happen. They need light and food. The lighting needs augmentation for the plants you have and the substrate won't provide sufficient nutrients. In fact laterite takes time before iron is released properly. Check out the section in the forum regarding fertilizing to better understand the basics. I initially started using only a all in one plant food with CO2 booster. My plants existed but never thrived. With an inert substrate we have to introduce the nutrients either into the water or substrate.