Well keep in mind that stocking is relative.
You only have as much beneficial bacteria as your ammonia and nitrite allows.
As you add more, you add more need for more beneficial bacteria right?
Due to this, you want to add them slowly to allow the beneficial bacteria to get to the strength it needs for the betta, then the snail, then the next snail, then the shrimp.
Snail and shrimps also do better in tanks where the filter is established so you would be best off waiting for the filter to catch up to each individual.
If you add them all at once, the bacteria can be overwhelmed and your livestock will be heavily stress and live a lot shorter (or die in a relatively short amount of time)
I guess you could as long as you do 33% water changes every day (or every other) so 1 gallon every day or two.
I wouldn't recommend it though. I suggest you build a good habit and stock slowly to air on the side of caution.