California insurance officials are seeking new limits on a controversial form of health coverage insurers are selling to small employers. At issue is a new type of self-insurance for small businesses with as few as 25 workers. Critics said insurers such as Cigna Corp. are using these new plans to game the system and cherry-pick companies with healthier workers. They said this could undermine a key goal of the federal Affordable Care Act to lower premiums by pooling together more healthy and sick Americans into insurance exchanges. Premiums could continue to escalate without a diverse pool of consumers.