Hi and welcome to the forum
Bettas come from soft acid water (GH
below 100ppm, pH below 7.0).
Platies come from medium hard water (GH
around 200ppm, pH above 7.0).
These fishes come from different environments with different water chemistry. As such they should not be housed together.
Common Bettas are Betta splendens (Siamese fighting fish). It's all in the name. There are two main groups of Bettas, the Betta splendens group and the Betta pugnax group. The B. splendens group are highly territorial towards other Bettas and other fish that are brightly coloured and come into their territory. They are bubblenest builders and should be kept on their own. The Betta pugnax group are mouth brooding Bettas and are fine in groups and with other fishes that don't bully them.
A 5 gallon tank is an issue for livebearing fishes like platies, guppies, mollies & swordtails. These fish regularly (about once a month) produce a batch of babies. They don't need a male in the tank to do it either. If you have 2 females giving birth to 50 young every month, a 5 gallon tank is going to become heavily overstocked, very quickly.
You need a picture on the back of the tank to make the fish feel more comfortable. You can buy aquarium backings from any pet shop or online, or use some coloured card or a plastic bin liner. Just tape them to the outside on the back of the tank.
The white gravel is going to reflect light and stress the fish. Bettas naturally occur in shady areas and have lots of floating plants to provide protection from aerial predators and bright light. Some floating plants would help reduce the light reflecting off the substrate and give the female Betta somewhere to sleep. Water Sprite (Ceratopteris thalictroides/ cornuta) is one of the best floating plants. It grows well on the surface but can also be planted in the gravel. It has branches and big leaves that will shade the tank, give the female somewhere to rest, and reduce algae problems. 2 or 3 plants should be ample for that tank.
If you had a male betta splendens, I would suggest removing the plastic plants and any hard or sharp objects in the tank. These items can cut or tear the long fins found on male fish. Live plants are better for male Betta tanks. You have a female or shortfin male, so the plastic can remain.
TURNING LIGHTS ON AND OFF
Stress from tank lights coming on when the room is dark can be an issue. Fish don't have eyelids and don't tolerate going from complete dark to bright light (or vice versa) instantly. This is made worse if there are no floating plants or you use a light substrate in the aquarium.
In the morning open the curtains or turn the room light on at least 30 minutes (or more) before turning the tank light on. This will reduce the stress on the fish and they won't go from a dark tank to a bright tank instantly.
At night turn the room light on and then turn the tank light off. Wait at least 30 minutes (or more) before turning the room light out. This allows the fish to settle down for the night instead of going from a brightly lit tank to complete darkness instantly.
Try to have the lights on at the same time each day. Use a timer if possible.
If you don't have live plants in the tank, you only need the light on for a few hours in the evening. You might turn them on at 4 or 5pm and off at 9pm.
If you do have live plants in the tank, you can have the lights on for 8-16 hours a day but the fish and plants need 8 hours of darkness to rest. Most people with live plants in their aquarium will have the lights on for 8-12 hours a day.
The glass surfing is common in new fish that have just been added to a tank that is clean. They are trying to work out how big the tank is and where everything is. If fish start glass surfing after you do something to the tank (ie: add new ornaments or fish), that is stress related. Your female Betta is stressed out. More plants, in particular floating plants, should help reduce the stress, as should a picture on the back. If it continues after you add more plants and a picture, then the fish is not happy having tank mates.
Did the shop tell you about the filter cycle and how to clean the filter?