First, red sex link is a BROAD term. It only tells you that the chicks can be sexed at hatch, by color, nothing else. Mostly, it is the highly specialized, genetically selected, often commercial hens sold to the public that people associate with issues in laying. The truth is, even these sex links lay like crazy, if only for a few years.
What you propose is not such a commercial pullet, but simply a very well bred offspring that you can sex link at hatch. That's all. They should be excellent birds, and no better and no worse, in terms of issues, than their parent stock. I'd love to have some!!!!
All chickens can lay soft eggs. Soft eggs are caused by a lack of calcium in there diet. I have had Rsl Hens and they are the best brown egg layers hands down. My Rsl hens lay at about 90-95%. I keep between 60-70 Rsl hens on average and I get between 50-65 eggs a day. I either fed them Purina or Nutrina 16% with free choice crushed up egg shells in a small feeder. I never have had soft egg shells. Try feeding them more calcium. My rsl are from a friends birds. He crosses hatchery RIR Roo over hatchery Delawares and they are great layers. He is an Npip flock and they are his favorite birds! I'm not sure how well of a layer they will be fro crossing heritage birds but its worth a try!