Agree with Grimmjow on both points. A timer for the lights is good, either built in or one you can purchase at a hardware store. Fish and plants need total darkness, that means no room lights either, for their rest period which should be 8-10 hours daily (or nightly I guess
). Over a 75g, presumably a 4-foot tank, I would suggest a dual tube T8 fixture. T5 HO with two tubes will be far too intense [unless you are going high-tech planted, a very different issue]. Dual T8 tubes will allow you the option of combining two different tubes for good plant light and good colour rendition.
Which "Twig" is it? The one in our profiles [click on shaded names to see profiles, example Twig Catfish] is Farlowella vittata, the smaller species. As noted in the profile, under "Origin," currents are not necessary, beyond normal water movement in a 75g with a canister filter. I have this species and even spawned them.
The nice thing about canisters is being able to direct the outflow against the end wall via spigot or a spraybar, and thus create a weaker current down and through the tank.
On the substrate, mixing gravel and sand is not in my view a good idea. If you want to have areas of just gravel and other areas of just sand, you will need some form of divider, and water movement and fish actions tend to dislodge these unless they are somehow permanently attached (siliconed to the glass). If you mean mixing together gravel and sand, also not good because the sand being smaller will drop to the bottom, and the danger here is that it will not bet moved about enough and compact. For planted tanks the best substrate is either small-grain gravel (each grain 1-3 mm) in a dark shade (black, dark brown, buff mixed, or dark gray mix) or an enriched substrate like Flourite or Eco-complete.
The above is general to get you started thinking, we can elaborate on points. And, welcome to Tropical Fish Keeping forum.