Small Business Owner: Yes.
Freelance Writer: Yes.
Subsistence Farmer: No.
Capitalism is the use to the private sector to grow the economy.
This means that the private sector has to be producing a good or service that can be sold in the marketplace to benefit society.
Let us take a look at your examples.
A small business owner is, by definition, owning a business that is producing some good or service for which there is a market. One example would be a barber shop or a "mom and pop" restaurant. It does not matter if the venture is making millions of dollars or just making ends meet. It does not even matter if they are the only employee. The business is making a good or service that is demanded, and they are fulfilling that demand.
A freelance writer is also producing a good or service that is demanded by others. This could be written articles, books, essays, etc. or the service of taking notes and transcribing words. Again, it does not matter the payment received for this good or service or if the freelance writer is a loner.
However, the subsistence farmer is different. By definition, a subsistence farmer is only producing enough food to keep themselves alive; they are not producing any food to sell to anyone else. The farmer (presumably) has no other source of income and is forced to produce all other items on their own. So, although they are innovative, the subsistence farmer is not a capitalist.