120g Low Tech setup

I am in the process of setting up a 120g (48x24x24) FW planted tank and want to make sure I am doing setting myself up for a good experience.

I do not want to deal with CO2 systems, at least not yet, so I will be starting out with a low tech system. I realize things wont happen as fast, but thats probably for the best my first time around. Naturally I will need to get plants that can thrive in a lower CO2 environment.

The thing I am reading the most conflicting information on is lighting. Some say light as in a CO2 system, others say light less to slow algae growth. I am planning on using T5 HO bulbs, but am not sure how many. I have seen people say 2 bulbs is sufficient for a 120, some say 4, others 6-8. I am looking at a Tek fixture right now that lets you switch from 4-8 bulbs so maybe that would allow me the most flexiblity in terms of upgrading to a CO2 system later, where it is pretty unanimous that you need 6-8 bulbs for a 120g.

For the substrate, I am thinking 2" of EcoComplete, 3" in back to 1" in the front, maybe with an inch of black gravel on top to save some money.

Anything else I need to know before I get started?

Thanks

When I add CO2 later (I know this will happen eventually, probably within the next 6-12 months), how many lights would I need? I see the fixtures at home depot where you an get 1,2,4, or 6 T5HO bulb fixtures. Does anyone know if they will work without all the bulbs installed? Could I buy the 6 bulb fixture and just add as many bulbs as I need, spaced accordingly?

Ordered a 4 bulb HO T5 light that has a switch to go from 0-2-4 lights! Should give me all the wiggle room I need.

I have been starting to think about decor in the tank and want to get rocks and some wood. Considering that the rocks will be heavy, especially if I stack a few to make caves and some visual depth, I am feeling hesitant about them being supported by substrate alone. I can just imagine myself (or a shrimp or fish) vacuuming the substrate, move too much and the rocks finding themselves sitting on the tempered glass bottom or falling. The I go to pick them up, slide them a bit, and a 20 lbs rock is being balanced on a point. I may just be paranoid, and if I was setting up in a concrete basement of my own home, I would be less so, but in a 4th floor carpeted apartment, I want to do everything I can to minimize possible risk. Thinking of laying egg crate on the bottom of the tank so that the weight of the rocks is distributed, and more importantly, the rocks will ever contact the glass, just the plastic. The substrate will be 2-3", while the egg crate is only about 0.3". Will that negatively affect my plant roots? Is the addition of egg crate a good idea? Or perhaps instead of a single sheet covering the entire base, cut it to fit under the rocks to ensure that the rest of the substrate can be circulated to prevent dead spots?