7 harlequin rasboras will be fine for a long time but when they get full grown they are a full 2 inches. They look bigger then all other nano fish that I have. So it may become overstocked and require more frequent water changes as they get older. If it was me I would do the betta and 5 harlequin rasboras. But I agree with start with the betta then add 2 or 3 and then 2 or 3 more! You should be fine!
You could try out some live plants like Java ferns or Anubias as well. Rhizome plants don’t need to go into the gravel so you can grow them regardless of your substrate. And those plants are sold at any pet store! Those will help the tank a lot!