Bill, thank you for your response. And yes, I agree with you that you can have a traditional IRA, as well as roth IRA in addition to your solo 401k. The only issue is that income limits to make tax-deductible contributions depend on whether you are covered by what IRS calls "employer plan". So, is a solo 401k based on someone's small self-employment income, covered under definition of "employer plan"?
Sorry, for wrong link format.
Check table 1.2 and 1.3 at this page:
http://www.irs.gov/publications/p590/ch01.html#en_US_2013_publink1000230467
So, if your solo 401k is same as "employer plan", for a single, you can make fully tax deductible IRA contribution only if your modified AGI is less than 59k. However, if you don't have 401k with W2 employer and your solo 401k is not counted as "employer plan" then, as per Table 1.3, you can make a tax deductible IRA contribution, regardless of how high your modified AGI is, if you are single / HoH (and income limit of 178k for married filing jointly)!
If solo(k) is counted as "employer plan" basically, your retirement contributions can only go to solo 401k and that is limited to probably approx 25% of your schedule C income only.
This IRS page tries to clarify employer plan but it is not clear to me if solo 401k fits the bill.
http://www.irs.gov/Retirement-Plans/Are-You-Covered-by-an-Employer%27s-Retirement-Plan%3F