This might be better in the Tax Forum. Regarding the side business income, you definitely can put that into a Solo 401(k), both Vanguard and Fidelity are good. You should be able to put almost all of that income in the Solo 401(k) after payroll taxes are taken into account. However, I'm not sure if there is a limit to 401K contributions per person, because you already have a 401(k) at your regular job. This means adding your wife as a partner to your side business would then make sense.
Regarding the S Corp for your side business: that is one way to avoid having to pay some Medicare/Social security taxes. There's threads on the Tax forum regarding this, and what is considered reasonable income as an employee of the S Corp. Obviously you don't want to do anything too crazy cause the IRS will state you're just trying to avoid payroll taxes.
What all your questions come down to is understanding tax law, you can learn a lot more of this on your own, but you definitely should consult with a CPA regarding the goals of forming the S Corp. for your side business and to make sure you're not making the mistake of going beyond maximum 401(K) contributions allowed. Other questions are will adding your wife as a partner to the side business help with making more of these 401(k) contributions. Your traditional IRA contributions also have income limits. So you need some CPA help with this thicket of issues.
You talk about a small business tax, and I'm not sure if you're talking about your state tax on small business property, or state tax on a C-corporation. Your S-Corporation income is passed onto you and you merely pay federal tax on it, you don't have to pay payroll tax on that portion of income.
