The 1/3 of a bottle of the Tetra Safe start or the Plus says it is good for what size tank? I am thinking it was for a 15 gal. If so 1/3 is enough for the below.
If you have enough of it, you should be able to add it to the tank and put the fish in and be OK. At worst, you may need to change a bit of water. I am assuming it is a 5 gal tank. You should not add any ammonia for the above. The fish will supply it.
The problem with getting plants at pet stores and even some fish stores is their condition. Plants that are not in good shape may not perform as expected right away. But if you can get a healthy bunch of stem plants you can float. 3-5 stems and they would help. You can also plant the stems in the substrate. But plants need light and nighttime as well. Indirect light would be OK. Direct sunlight will likely make algae.
I am used to working with Dr. Tim's One and Only. The two products are almost identical. Dr. Tim discovered what was in them 20+ years ago. He and Tetra (via their parent company) share the patents.
There is one thing you should know about bacterial starters. The live bacteria are pretty hardy. But there are two ways to kill them pretty fast. One is to freeze them. Not an issue now. The other is to let them get to warm. Much over 100F for any time can do them in. So how they are shipped and stored once they leave the manufacturer matters. If your bottle got really hot along the way to your tank, that could have killed some or all of them.
I buy by my Dr. Tim's on Amazon. Our home has Amazon prime and we can get stuff here very fast if we have to for little or no extra cost.
Here are their directions for use for a larger size- I have added some comments in red text:
Shake Well.
To start new aquariums, add entire bottle to aquariums up to
X gallons.
(The smallest bottle is for 15 gals. so 1/3 should do 5 gal.) Aquarium is then ready for fish. Consult your retailer for advice on type and number of fish appropriate for your aquarium.
(You have 1 betta all is OK.)
For healthy maintenance, add one teaspoon (5ml) for every 10 gallons when adding fish, after water changes or after replacing filter media.
(Ignore this.)
Tank Preparation: Condition tap water with Tetra AquaSafe®. Maintain a constant, appropriate water temperature in your aquarium.
(Any dechlor will do.)
Note: Live Bacteria, do not freeze or expose to heat. (I already said this )
INGREDIENTS: Purified water, Proprietary strains of: Nitrosomonas, Nitrosospira and Nitrospira.
Further suggestions. Do this soon.
BE SURE TO START WITH 0 AMMONIA IN THE TANK WATER!!!!
Test for Ammonia and Nitrite in 24 hours and every day before yo go away.
If ammonia is .25 ppm or lees, do nothing. If ammonia is over that change enough water to get it under .50 ppm and .25 is better.
If nitrite is present, take a bit ot water from the tank into a clean glass/cup. Put a pinch of plain old salt in the water, stir and add it to the tank. The chloride in the salt blocks nitrite for entering a fish. You need very little salt in your tank to combat nitrite.
My old brain cannot remember if you have your own well and do not use dechlor. If not, if you have to change some water, dechlor only the new water at the recommended level for your brand. Do not over dose it.
Feed as sparingly as possible. Your fish can go for days without food and do OK. If you plan to leave food for somebody to feed while you are awy. Use a small piece of aluminum foils and wrap up the exacy amout to feed in it, This way all the person needs to do is open the foil and add the food.