You say you got a "paycheck". Did the company withhold "social security taxes" ? If they did then they treated you as an employee and the 15.3 % self-employment tax does not apply to the income earned at the company. If the company did not withhold social security taxes, then they are treating you as a freelancer or as the IRS calls it an Independent Contractor. However, the other freelance income is subject to the 15.3% if it is more than $400.

Both income sources is subject to "income taxes" regardless of who paid you. Anyone that pays you are taking the expense as a write-off. That is their business expense. The "guy" is required to get your SS# if the amount he pays you will exceed $600 for the year. If he does not get your SS# and report the payments to you and the IRS he will loose the deduction on audit and be fined $50 per person up to $100,000 fine.

Here is a reference that requires you to give your SS#:

2004 IRS Publication 17, ?Your Federal Income Tax?, page 24:

>>Failure to supply social security number. If
you do not include your SSN or the SSN of
another person where required on a return,
statement, or other document, you will be sub-
ject to a penalty of $50 for each failure. You will
also be subject to a penalty of $50 if you do not
give your SSN to another person when it is
required on a return, statement, or other document.
For example, if you have a bank account that
earns interest, you must give your SSN to the
bank. The number must be shown on the Form
1099?INT or other statement the bank sends
you. If you do not give the bank your SSN, you
will be subject to the $50 penalty. (You also may
be subject to ?backup? withholding of income
tax. See chapter 5.)
You will not have to pay the penalty if you are
able to show that the failure was due to reasona-
ble cause and not willful neglect. <<

http://www.irs.gov/pub/irs-pdf/p17.pdf

Sorry but I only answer questions here on the forum.