It's ok to start planning what your going to feed in advance, but it won't guarantee the gliders you get will eat the diet you choose.
The type of bird food and store bought food your speaking of is not ideal for gliders and will not nutritionally provide all they need. Gliders should not be fed seeds and nuts like birds or rodents, their teeth are not designed to break them down enough to digest them.
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First, I would ask the breeder what diet they are currently being fed, if it's a well known widely used diet ( Bml, Critter love hpw, even Tpg ) you might want to keep them on that.
If the diet they are currently on isn't well known or widely used then you'd want to look at other diets. The two I recommend are
Bml
http://www.bmldiet.com/bml-recipe.html
and Critter Love
https://www.critterlove.com/
Between the two Bml is easier to make and understand for new owners, in my opinion.
They both have their pros and cons... Bml you can find all the ingredients locally, Critter Love you have to order the staple online. Bml takes a little longer to prepare nightly, Critter Love takes a lot longer to prepare in advance. Bml takes up little freezer space, critter love can take up a lot of freezer space depending on how many gliders you have. Both diets have been widely used for years and have been tested.
Each diet has their own fruits and veg that need to be fed with the staple, so be sure to read each diet instructions and feeding directions and follow
them.