No!!!! No goldfish!!!! They grow fast, eat the other fish, and are "dirty"....can't keep the water clean. NO goldfish.
Beta fish are wonderful and smart. I had a 10 gallon tank and put my beta in there with some other fish. He did great. No issues....just not another beta.
Tetras are "cute" and very pretty colors. I also had an algae eater who had the greatest personality.
Speaking of personalities....your beta will have one. He gets to know "you" and when a stranger walks in he will actually hide from them. I love the betas....but, it's sad that they are sold in those ugly cups of blue water. So humiliating for such a lovely fish.
If you get guppies, make sure there are no females mixed in, or you will have nothing but fish in your tank in a few weeks. Besides, it's really sad watching the males constantly harass the females....the poor girls can't get a moment's peace.
You can get heaters really cheap, and a good filter will last you forever, you'll just have to change out the filter. I never did water changes....my water would simply evaporate and I would add more.
Get an aerator...makes pretty bubbles and oxygenates the water.
It's a great hobby.
Just be sure, if you get live plants, there aren't any snails hiding on them. UGH!@#!$!@!!!!! I did that once. I loved the plants and even the cute little snail was interesting....until I realized he could propagate all by himself...and before I knew it the whole tank was covered in these ugly little brown specks. Seriously....in the thousands...and they messed the water up terribly! They pooped all the time. I tried getting rid of them. Freaked the fish out by catching them....then washed all the decorations, left them to bake in the hot summer sun. Rinsed the tank, bleached it....rinsed it and left it in the sun. I got new pebbles, filled the whole thing back up....let the water cure....put the fish back....and in a month had about another 100 snails. Yuck!!! It only takes one.....