Assuming you get the right betta, harlequin rasboras are peaceful enough not to chase the betta. They are also from the same region so they require very similar water quality. However, I will warn you that you must have a cycled back-up tank for the betta if it doesn't work out. There is always the risk that the betta could attack the other fish, and the other fish may nip the betta. You also have to take into consideration feeding habits. My betta fish are always piggies, but they also should not eat flake food. If you have a betta in a community tank you need to think about how you will feed the fish. Best thing would be to feed stuff like blood worms, brine shrimp, and daphnia most days so that while the betta hunts after that, you will be more free to feed vegetable matter in the form of veggie flakes or what have you for the rest of your fish. You can also distract busy fish with flakes and drop pellets right in front of the betta, but this requires a bit of skill and good timing. You will also need to plant the tank so that everyone can have some hiding spaces if needed.
Having said all of that, a community tank can happen with betta fish. It would be even more possible if you could get your hands on some of the non-splendens betta which are actually typically more peaceful. I would not worry about adding another centerpiece though. I'd think that a nice calm school plus a stunning crowntail or OHM would be very attractive.
