A pre-fab, store bought divider will, indeed work in a 5 gallon tank, with some slight modifications. I have a top fin divider (the middle replaced with craft/crochet mesh (.50 cents)) in both my hex and the tank above. It needed slight cutting, but the replaced craft mesh made it alright if I messed up, its only .50 cents!
Other substrates to consider are marbles (flat bottomed aquarium marbles!), small river rocks (sold at stores) and sand. I suggest you also purchase a siphon to make water changes easier. Itll get the poo off the bottom AND take water out, making things easier for you.
Btw- that filter's gonna need a baffle. Found that out last night D:
Not gonna lie, bowl buddies kind of suck, haha. Most liquid conditioners are the same, save ones like Prime that remove ammonia, too. Anything that removed chlorine is good, though look for ones that also remove chloramine and heavy metals. Look for one appropriate for dosing your tank- you dont want one that needs 1tsp/ 20 gallons! Find one that goes in drops for .5 or 1 gallons/drop or something that doses 1 tsp/2 gallons. I use top fin.
Fake plants are fine, but take care they are not sharp. If they snag or rip a pair of panty hose, theyre no good. Silk is usually great. Mugs are fine, though make sure they have enough room to swim, dont let it take up too much space! Terra cotta pots are also a good idea, though if they have a hole on the bottom, it NEEDS to be sealed up!
Good live plants are anachairs, java fern, anubias and water sprite in your tank. Other options open up depending on your lighting and substrate options. For now, all but java fern can just float around, and none need high light ;)
Try to get a tubular >15w florescent light to help plants grow!