First of all, your question regarding Zecco.
I have an account with Zecco. A small starter one, to get the feel for them. At the moment I hold some GMR and F (Both long term) with them. There's no real "catch" although there are a couple of contingencies. You can't day trade with Zecco. If they catch you daytrading with them (according to the SEC's rules), then they'll probably lock your account up. But that's ok, because for a newbie to try day-trading - is well . . . suicide.
The free trades last for 40 trades a month, or 10 per day. After that, it's $3.50 a trade; which is still very reasonable. Heck, I don't think I've ever performed 10 / 40 in my entire trading / investing career.
They basically make their money by the interest on margin accounts and options trading - and they are keeping their costs small with a different strategy for advertising.
Second question. Your example. Yes. If you bought Boeing at $100 per share, there would be no fees. Just that, you'd buy it at $100.00 per share. You could sell it the next day if it hit $102.00, and take your profit. No fees. Up to 40 times per month, or 10 times per day. No problems.
$300.00 isn't a bad amount to spend for a starter account. It'll help you get your feet wet.
Whatever you do, I suggest you read, read, read. Read about Money Management principles (The most important concept to a trader, and to an investor). Read about P/E's, and fundamental analysis. The difference between investing, and trading. Read about Seasonal tendencies. Read about Chart patterns. Read about technical analysis. Dividend investing. As much as you can get your hands on, but ESPECIALLY Money Management principles. I've learned not to listen to folks who tell you only one of those is "the only thing you need" as if it's the holy grail of trading. Some people do fine with just one - and that's no problem (Generally, those people are really good at money management principles). I've found however, that it's more advantageous to combine them all.