Rather than keep answering similar questions, I'll be plain . . .
Yes, you can add your betta now.
You can add Stress Zyme if you want, but it's not needed.
When your betta is in the tank, do a 50% water change whenever the ammonia goes over 0.25ppm.
If you don't have a test kit, do 50% changes twice a week.
After 8 weeks, change the schedule to one 50% change a week
Every 2-4 weeks, swish your filter in tank water. Don't use tap water and don't scrub it.
Use a water conditioner whenever you do a water change.
If you do all that, then your betta should enjoy good water quality.