As far as the carbon filter, a number of people with planted tanks seem to feel the carbon will remove nutrients that the plants need, so they avoid the carbon.
All I can say is I use carbon filters and have plants that are doing very well. I've used carbon for a long time. I think the carbon helps a lot with the water clarity, and I prefer clear, clean water. The only time I would remove the carbon would be if I added medications. The carbon will filter out medications you might add to your tank.
I had a carbon filter for a while but IMO its a waste.
Your pH is actually REALLY good. Bettas prefer slightly acidic water with a pH from 6.5-7 being ideal. They can tolerate higher pH but it can cause fin curling. That pH would be excellent for keeping a CT.
Turkey basters work well but I wouldn't pick up debris while cycling, wait until after your cycled.
IMO your tank would be fine. It would be fully stocked and you would need to watch your prams but I think with a healthy water change schedule it could work fine. I would suggest planting the tank to help with absorbing wastes and producing oxygen.
Like they said, bettas like acidic water, and yes, you can use carbon once the cycle is complete.
I would reconsider your stocking plans, though. Otoclinus catfish are very sensitive fish that only eat a small range of algae, mostly diatoms. Many of them are starving in the pet store before you even buy them, which is why they're only recommended for established, stable tanks with the right amount and the right variety of algae available. They're probably not the best choice for you, especially if the tank is new.
Have you considered, perhaps, a school of pygmy cories and a couple of african dwarf frogs? Both are adorable and have tons of personality, and they won't harm the betta. They also are unlikely to be attacked by the betta and don't have the swimming space needs that the rasboras would. Just some food for thought. :)
Adastra: My petsmart doesn't sell pygmy cories, and only occasionally has the regular cories in stock, same for the ADFs, but if the store has any, I'll consider it :)
5) I have this old tree in my backyard that's been dead for some years and has finally been cut down. I'm not sure what type of tree it is, but even if it was a softwood tree, all the sap is gone. If I properly boiled a piece of it, could I use it as driftwood in my tank?
I wouldn't, if even if the sap is dried out, it could still be harmful if the tree was a pine or cedar tree or similar. I don't think it would be possible to get the toxins out of it. Most Petco/Petsmarts sell bags of mopani wood for a couple of bucks. You get about 4 pieces in a range of sizes. Petco also sells pre-soaked malaysian driftwood--it's a little more expensive but it won't cloud your water and you'd only really need to boil it once just to sterilize it.
You should see if there are any local aquarium clubs or societies in your area. They are more common than you might think, and often you can get a large variety of fish from such organizations for a more than fair price.