If using the Critter Love diets (formerly HPW), you will not need added supplements. It is crucial to follow the diets exactly how directed, including serving. I know that CL diets have been formulated for you to provide correct balance of nutrients, including calcium. CL now has specific salad formulas that provide this balance. The BML diet, is also formulated as directed to be balanced, the "must have" fruit and veggie list balances the high calcium in the nectar formula.
I have heard of the insectivore food, but am not knowledgeable in diets using it. I have read through the various diets soon after getting my first gliders (originally I had them on PP diet, though the gliders themselves were not originally from PP). I decided on the HPW (now CL) diet from the beginning and have only experimented with those and similar diets. So unfortunately, I do not have much knowledge on any others.
Once you decide on a specific diet, and you want to offer some insectivore as a treat if they like it, then you can offer small quantities on occasions. Some offer mealworms along with their diets as treats, or even some other types of treats. I personally have a trio, and there are only 3 different types of treats that I have offered that all three like, lol. 1. freeze dried mealies- I get Camilies vita-mealies freeze dried and offer on nail trim days and here and there. 2. Pine nuts-I purchase small bags (they last a while) and offer on occasions while I have them in stock, but I don't keep an ongoing supply, so it's on & off. 3. They all like the Pet Pro Happy Glider fruit flavor pellets, so that's what I usually keep in their forage toy inside their cage. Pellets are not a staple food, but some like to offer them. Good quality pellets are not bad for gliders as some have said, but only if not used as the staple. My glider's vet prefers the HPW Original diet (before name changed to CLO). She recommended using these pellets on the side. I prefer using them in their forage toys.
Have you asked your vet why they like the insectivore diet?