Is it a good idea not only to diversify among various mutual funds, but also among different companies that sell mutual funds?
There's no need to do that. Find one good broker you're comfortable with, and who has the heart of a teacher. You want to know what's going on with your money, and finding someone who can explain it well and help you understand the details is a must!
Just make sure your broker is not directly connected to the mutual fund. You don't want someone with a vested interest. What you're looking for here is someone who can objectively connect you to a good mutual fund with a solid five- to 10-year track record.