Young Macerick - are you already servicing SME's in your current firm?
There is a market in the SME sector but day rates are low, owners are mean, projects are shorter and teams smaller. If you want to live a cosy life servicing local SMEs then you can probably make a six figure wage, but you can't get rich by building up a 50-100 person consultancy in this way.
To your original question. Yes, all firms started this way, even Accebture, so it can be done.
The challenge in starting your own firm is selling the first pieces of work, then delivering them while you simultaneously sell the second an third piece. That's why so many 50 man consultancies are still massively reliant on a small number of clients (or even single projects!) for their income.