Q: Can I get reliable financial data from a small company in Spain?
A: to be direct: most of the time, no, you can't.
Most small companies only have their bookkeeping done to comply with their obligations to the tax authorities and make sure they pay the minimum amount - if any - of taxes, not to get a financial report on how the company is doing.
There is a famous term in Spain to refer to the not-declared sales and profit numbers: "caja B" (the B-box). Here is where the real profitability of small companies usually shows up.
It takes experience, common sense and knowledge of often used tricks to calculate the real sales and profit figures and how to square these numbers with the officially declared sales and profit. Only then can you make a real evaluation on how a Spanish company is really doing.