You're trying...that's for sure. But they absolutely need an approved diet. The ones you've mentioned are good...none are better than another, it's more personal preference and what you're capable of providing...but they need to be on ONE...period.
I wouldn't give the ensure. The quantities of vitmins and minerals are not in any kind of balance of what a glider needs...and could lead to an overdose. Sprinkling the vitamins as you are can also lead to the same issue.
Get those babies on a good diet asap. Especially since you are noticing possible malnutrition or failure to fully thrive.
Best of luck!!!! Stay vigilant!!!