I've made 1/2" mesh hoppers for mine also, and I use a mix of Mazuri 6F and dog food. The dog food is whatever happens to expire at my clinic (I check for red dyes though), so it's mostly just supplemental, but the large-breed formulas have bigger pieces that don't fall through. I don't think they would eat it if it was in 1/4", just because it's so much harder for them.
You can either find bigger kibble, or put it in a dish. You have more waste with a dish though.
If you're having difficulty with weight gain, make sure you feeding 6F, not the regular Mazuri (I assume you are breeding?). The plain Mazuri doesn't seem to have enough calories to compensate for pregnant/nursing females and quickly growing pups, even though it says it's for all life stages. Also, you might want to think about checking for internal parasites by taking a stool sample to a vet. Most vets will require seeing at least one rat in order to prescribe meds, but they'll likely use something you can get over the counter and if you let them know you have a large amount you might be treating, they'll give you the dosages.
In my travels on the internet,, I've come across something that said generally how many pellets a rat will eat a day, but I can't find it again.
You do want to just fill it up. They'll regulate themselves, and you definitely don't want them to be hungry.
EDIT: Actually, I did just find something. Mazuri's website says in regards to the regular food "Rats: Adult will eat 16 to 25 grams of feed per day. Mice: Adult will eat 4 to 5 grams of feed per day daily." It'd probably be similar with the 6F
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