How can the co-founders that hold a minority of the shares be protected against:
The way to protect minority (or any) stockholders against dilution is with a written non-dilution agreement.
Basically such an agreement could state:
1) Existing stockholders have a right of first refusal to purchase any additional shares being offered by the company.
2) A prohibition on the issuance of new shares without the prior consent of a super majority (2/3, 3/4 or whatever) of the existing shareholders.
You should be aware that such an agreement, while serving your desires, may or may not be good for the company. In general they are frowned upon.
Wikipedia has a nice article on stock dilution, which can take many forms.
Here is a more in depth article: A primer on anti-dilution provisions written by a lawyer.
Here is a sample agreement.
And a discussion on Quora See also this answer Can non-dilutable stock (or equivalent) be issued?