What I would do, if you don't already have some, buy a small amount of SCI and experiment. Wherever you've been getting your SLSa, should also have SCI. Important: you'll want the powder, not the noodles, since for this application the noodles aren't ideal (they don't dissolve in water easily without being abraded, i.e. being rubbed on a wet washcloth/flannel or on wet hair).
You'll want to make one with SLSa, one with SCI, and a few with varying ratios of a mix of both. Keep in mind, the other ingredients in your products will affect its ability to lather, so if you still aren't getting the mountains of foam you're after, you can look into your other ingredients to see what you can tweak to maximize foam.
And as always, as KiwiMoose already brought up, there are maximum limits for every ingredient. Check with the vendor or manufacturer to make sure you aren't exceeding safe limits.