Considering they were nutritionally deprived and physically stressed during their earlier development, this delay in laying is normal. To help them catch up, I would put them on a higher protein ration, like Purina Flock Raiser (20% protein). You'll have less feed waste if you use pellets rather than crumbles. You can use Flock Raiser long-term, as long as you provide supplemental calcium (like oyster shell) free-choice when they start laying. Personally, I like to give my hens a bit more protein than is found in layer ration, which is typically 16% protein. After they go through a bag of Flock Raiser, you could switch them to a 50:50 mix of Flock Raiser and layer ration to achieve 18% protein.
You did a very nice thing by adopting these birds, and I'm sure it won't be long before they start providing you with lots of eggs.
BTW, you do not need to provide light. In my experience, pullets that have been hatched in spring or summer will start laying in fall or early winter without any supplemental light, and will continue to lay through their first winter (again, without supplemental light). My RIR typically start laying around 24-25 weeks. Within a few months of starting to lay, they are each giving 6-7 eggs/week. If there are long-term effects from their earlier nutritional deficiencies, their rate of lay may be lower than this.