Betta + shrimps is not a very successful combination for most tanks as Bettas (except for those really rare examples) usually treat the shrimps as snacks.
IMO the ideal would be to keep a Betta in the 2.5g (if you can be at top of water quality maintenance), and some shrimps in the 1.5g.
I think you could start a pretty cool shrimp colony - not a lot of options when it comes to fish, but at least you are asking the question and not just dumping a goldfish in there. Here are a couple neat nano fish about .5 inches - 1 inch long, I do not recommend keeping them in the 1.5 gallon but if you watch your water parameters 2.5 could be okay.
you need at least 5:
Micro Glassfish (Danionella translucida)
Exclamation-Point Rasbora (Boraras uropthalmoides)
Dwarf Amber Barb (Barboides gracilis)
Strawberry Rasbora (Boraras naevus)
Phoenix Rasbora (Boraras merah)
Chili Rasbora (Boraras brigittae)
Golden Rasbora (Horadandia atukorali)
If I was able to find any of these then I'll get 5 of 'em and put them in the tank. if not then just 1 beta in 2.5g and like 5~6 shrimps in the 1.5g and that's it.
I'm making 2 new tanks soon.. 1 is being custom made with the cabinet(95g), and 1 I bought from a friend who's keeping it to store groceries and stuff lol (55g).
I'll put fishes in them since I want fishes so bad, and I'll keep those 2 simple.
thanks for all the replies guys.
+1 Don't overdo the decor or hardscape, it will take up too much room. And yes even though it is possible to keep the fish alive and healthy (even happy) one slip on a water change or too much food and it won't work out. Betta is the best option overall.
I guess beta fish it is, but I heard in nano tanks, when you have lot of plants around, in my case I'll have a full carpet, a java moss tree, and some medium sized red plant(forgot the name lol).
I heard having lot of plants will help keeping the water more stable even without weekly water changes?!.
I saw a video on youtube with a guy having a nano tank and he's like he haven't done water changes in 6 months, but the amount of plants in the tank is keeping it stable. he does top ups every now and then only.