It's not illegal to offer health benefits to just the top echelon of your company, but it may be difficult to find a broker/carrier who wants your business. That is, with fewer than 10 employees to begin with, to seek health insurance for only a very few will not make you a particularly attractive client for most health insurance brokers/carriers; and it will probably be pretty expensive for you, too.

Also, insurance brokers/carriers like to spread the risk across as many
eligible employees as possible. A better way to go might be to select the plan you think is a good one and offset the cost for the more valuable employees while not doing so for the lower-paid employees.

It's still not ideal, but it is legal. While the lower-paid employees might not be able to afford the good coverage, this would at least allow them the opportunity to obtain health insurance if they really want and need it.

A PEO (professional employer organization) might be an option for you at your size, too. That way, you could get the cost benefits of a larger group that offers a wide variety of health insurance programs. Your higher-paid employees can opt for a richer plan, while the lower-paid employees can obtain what they can afford. Also, with a PEO, an HSA (health savings account) option would likely be available, which might be advantageous to a segment of your work force as you continue to grow.

You can design other benefits to attract highly skilled employees, such as bonus programs, more vacation, free life insurance, paid parking, car allowances, seminars, professional memberships, education assistance,
free financial/retirement planning assistance, laptops, PDAs, etc. And you can look into programs that allow you to put aside more for their retirement than standard 401(k) or simple IRAs. These latter suggestions can also keep your highly skilled talent on board longer.