1. When I interviewed with a company in southern California, the CEO noted that there are different types of stock that get issued to the employees that are not as good. I don't know much about stocks, but it's an angle you will have to consider.
2. One thing I found is that if you're a founder, that makes you responsible for both the wins and losses for the company. So when it tanks, debt collectors will come for your stuff too. Where an employee just needs to worry about getting another job. This point alone means you'll have to carefully consider who you're getting in bed with.
3. The only reason I can find for not wanting to increase co-founders is for two purposes. One, it's another person who can flake off and start writing company checks. Two, you'll have to split profits again :)
4. I don't think so, don't quote me.
5. See 2. and 1. to weigh your options. Being a co-founder potentially has a bigger pay off but comes with responsibility.
6. See 5.
I say if you genuinely trust your future partners, to go for it. It's my understanding that if you want to be able to afford that sports car and have plenty of time off for those cool vacations; you either need to be at the top of a company or have enough money to somehow live off interest.