I think that most traders can avoid Social Security/FICA/Medicare Taxes if their "income" is based on cap gains, dividends, interest, etc. rather than earned or ordinary income. These taxes apply only to ordinary or earned income, not to capital gains.
Now its a separate issue as to how you set up your trading entity and whether that entity "pays you" for trading. If the entity passes the cap gains to you as cap gains, then those are NOT subject to to these taxes. If the entity passes the cap gains to you as ordinary income, then they ARE subject to self-employment tax. (I'm not sure how the Prop firms do it -- do they pass trading profits to the trader as "cap gains" or as "ordinary income" on the K-1???).
Before you think that avoiding Social Security/FICA/Medicare Taxes is great thing, you might want to look into its impact on retirement planning. The lack of ordinary income makes it hard to aggressively sock away funds into a retirement account where you can trade without any taxation (until you retire and start taking distributions). For example, I have a KEOGH account that lets me put about 20% of my self-employment earnings into a retirement account (and with the partnership agreement and "guaranteed payments to partners" I can convert cap gains into ordinary income). Once in the retirement account, the trading gains compound tax-free. When I ran simulations of this for my personal situation, I found that once I reach a certain level of profit generation(not quite there yet), I will be better off in the long-term if I convert some of those cap gains into ordinary income, pay self-employment taxes, sock away 20% into the retirement account, and trade tax-free inside that account. Over time, capital gains earned from trading untaxed capital gains in the retirement account make up for the initial hit from Social Security/FICA/Medicare Taxes.
Of course, there are other issues that impact whether this strategy will work for you. The lack of margin borrowing and presence of PDT(pattern daytrader) restrictions on the retirement accounts could make this strategy unusable to daytraders. Also spreading ones trading capital across multiple accounts can make trading harder. Since I swing-trade the long-side using end-of-day data and order-entry, the PDT restrictions and multiple account issues are not a problem for me.
As with all of matters related to the IRS, I will cover my a$$ by saying "Consult your tax professional" -- I'm just an ordinary guy who knows enough to be dangerous.
Happy trading,
-Traden4Alpha