Sorry to rain on your parade but IMHO I don't think this is an ideal setup for such a small tank. I recommend a 15 or 20 gallon for these fish. I own all of the species you mentioned and currently keep a large shoal of Cory's (false julii, panda, C. Leucomelas, and dwarf Cory's, with a dozen Rasboras in a 37 gallon. And I wouldn't use anything smaller than a 20 gallon for Cory's. I also own Bettas and they each showed signs of stress when cohabitating in my community tanks so they each live happily in their own tanks now. I also don't agree that they are strictly upper level fish. I know mine like to explore upper, middle and lower levels. But besides that I don't think a 10 gallon tank is the appropriate size tank to discuss "Levels", the tank just isn't tall enough in my opinion, a 20 high is far better for combining multi level fish.
I also run overstocked and compensate with extra filtration: my 37 gallon has an Aquaclear 70, a 40 gallon canister and 40 gallon sponge for a total of 150 gallon filtration. My 20 gallon has a total of 50 gallon filtration capacity. However over filtering a small 10 gallon in my opinion would also create a lot of water turbulence which I thought was the opposite of what bettas like.
I would keep the betta by itself and buy a 20 gallon, stock it with the fish you described and still have plenty of room to add the other fish suggested in previous comments.
"Level" is where a species generally establishes territory and spends a majority of its time. It has been my experience that no species spends 24/7 at one level. I don't believe I've seen anyone here aver Betta are strictly upper-level?
From experience, I find it is appropriate to discuss levels in 12" high tanks like a 10 or a 20 long. Maybe not back when I started and all one could get were larger species like Swords, Mollies, Cichlids, etc. But tiny Nano fish have rewritten the rules on stocking and how we view habitat.
As far as Chili Rasbora, scientific profiles and those by long-time, knowledgeable hobbyists/lecturers like Rachel O'Leary (msjinkzd on YouTube) all indicate a 10 gallon footprint is perfect for a shoal of 6-10.
When I had 10s, I over-filtered with no issues. I ran two TruAqua PFE-1 HOB (up to 15 gallons and 45 GPH) or one PFE-1 and one that is rated 80 GPH for up to 25 gallons. Both are adjustable.
I experimented and discovered that if one starts the filter out on a low setting and gradually increases the Betta will exercise in the output and begin to gain fin and peduncle strength. Mine even seem to enjoy swimming in and out of the bubble wands. With only the occasional HMPK, I have had a majority HM with exaggerated long fins.
From the above observations, I have never bought the argument that Betta don't "like" current. I believe that is our interpretation of the response of a weak fish to current. By "weak," I mean those raised in a current-less environment that suddenly find water actually moves! Condition a Betta and it will live happily in a current-filled tank. Also, mine seldom, if ever, surface to breathe.
Your opinion is much appreciated. It made me ask myself why I recommend as I do and to research my knowledge base for the answers.