Im not positive about the betta. Bettas are either hit or miss.. and since they are fighting fish, you could have an overly aggressive male who attacks your tetras.. or you could have a very docile one. I would suggest not getting the betta
Lets break this down by inches-
10 tetras at 2in each = 20 gals
3 cory cats at 2in each = 6 gals
1 dwarf gourami at 2in= 2
gal
TOTAL
gal: 28gal
Now cory cats can be anywhere from 2-3in... so you might come close to overstocking. Which Is *okay* if you keep an eye on your water parameters and do water changes like you should.
If you do buy those fish, I suggest adding the 10 tetras first... watch your water... and wait about a week or two.. and add a cory cat once every week... and then add the gourami
What tetras are you looking into? If you get 10 tetras, for your size tank you want ones that only get to be 2in or less.
Also, do you plan on buying water testing kits?