That terrarium carpet is just a waste of money. Go ahead and try millet if you want. I don't think it'll hurt your lizards if they eat a little. It seems like something that would rot too easily though. If I had a leopard gecko, I'd just use some kind of sand and a nice hide.
I've used sand for my LG's for almost 15 years and I've never had a problem. Now I won't use it on babies, I wait till they're about a year old. I also change my sand out when it gets dirty. Because my LG's go to the bathroom in the same spot, I really only have to change the sand it that area once a month. You just have to watch out for sand sticking to your geckos toes. I usually let them soak in some shallow warm water before and after they shed to get the sand and skin off the toes. It works fine for me IMO but not everyone likes it.
I agree with the both of you. Terrarium liner is a total waste of money. I really dont know what the paranoia with sand is about. They eat it in the wild sometimes on purpose, and im sure on accident a well, and theyre fine.
For my leos and my beardie, I have some funky 'desert landscaping sand' I think its called. It stays smooth about an inch to the top, and below that it hardens and cakes. Its the only sand Ive tried that holds anything like a burrow. My beardie has quite a time playin in it. And no, never had impactions
loco, I think the stuff you have is called 'excavator', I use it for my leos too, and so far I'm really pleased with it. Way better than that dusty vitasand crap that dyed all their toes blue. I never bothered to wet it/make burrows, I just gave the geckos plenty of stuff to hide in and around. But good to know that works. The millet stuff I agree would probably rot. Might be good for a desert tortoise but that's all I can think of.