So recently i got the money to upgrade to a mod set up that uses batteries and when my order came in I noticed that the batteries werent the same, i ordered 2 sets of 2 of the same ones from the website, the difference isnt in the volts or amperage but the mAh and i was wondering if anything bad would happen since the mod requires 4 batteries and 2 are 2500 mAh and the other 2 are 3000 mAh.
What a battery is rated at and how it actually performs are often two separate things especially when not buying the name brand of one of the few actual 18650 battery makers: Sony, LG, Panasonic, or Samsung but one of the discounters that rewrap their batteries under their own generic brand labels. Even name brand batteries sometimes perform slightly different from what they are rated at especially different mAh batteries from the same company.
Mooch the battery expert and tester released a chart that shows mfr rating, actual rating, and vaping rating.
https://www.e-cigarette-forum.com/forum/attachments/img_0775-jpg.689793/
You also might want to learn more about why people marry batteries that stay with each other for life. It is so identical batteries erode at the same rate. That won't happen if you are using batteries with different mAh ratings. It should be safe enough if actual and vaping ratings are the same but you will severely reduce the life of your batteries by doing what you are asking.
You should marry four identical batteries for your mod. Unless it is a Smok GX2/4, you won't have to deal with separate married pairs and quads.