Cories are great janitors, but they need extra food
Also they do need smoother types of gravel.
I've had bad luck with cories, most of mine don't last more than 6 months. I had cories that spawned one year, but I was totally unprepared.
Also with a betta in residence, you would have to make sure the betta's not helping himself to a portion of the shrimp pellets.
Those varieties of cories are usually not that common @ LFS and the ones I've seen usually start out at $3-$5 each.
- I think spending that money on essentials like a decent ADJUSTABLE 50-75W heater, & a decent filtration unit with some sort of flow control would be higher on the to do list.
- Also one of those API Master Water Test kits as well.
Cycle the filter which usually takes several weeks and once completed, then you can consider adding new fish.