Plaid was fine with creating a developer account for just a few bank accounts, they didn't have any minimum or anything.
One thing I ran into quickly when parsing my own bank's OFX was the truncation of the payee field, which made it extremely difficult to identify certain transactions. For example, I might get coffee at a Starbucks, but it would show up in the bank export as "STORE93423 CA LA STAR". I never determined if this was a limitation of the bank's software or simply that OFX didn't allow for enough characters. It'll be interesting to see if Plaid solves this.