Most US states are at-will employment, allowing anyone to be fired at any time, for no reason at all.

http://en.wikipedia.org/wiki/At-will_employment

Fun fact, many anti-discrimination laws in the US don't apply to companies with less than 15 employees.

Even if it's not allowed, you could always hire the person on a contract basis for the trial period. Then offer a full employment agreement after that.

I'm Canadian, but here most, almost all, companies have a 3 month window in which a new hire can be let go without severance.

This is allowed in the Netherlands, and I'd wager in most European countries.

Not in California. It's called contract-to-hire usually.