Would a jack russel terrier be able to get in this? she is 18 lbs..

No she won't be able to. Here is a link that explains more and has a video you can watch. http://felinesonly.ca/

I have a 6lb chihuahua. Pretty much the size of a cat. He’s a food brat, he gets very selfish. Would he be able to get to the food due to his size?

Hello and thank you for your interest in our product. Your chihuahua would have the advantage over a bigger dog in that he's closer down to the food. However he doesn't have the shoulder flexibility of a cat, so the eating position wouldn't be as comfortable.. You would have to stand by him when you first put the dish down and use verbal commands as well to reinforce his decision to stay away. IF you decide to purchase the dish and have any concerns, please don't hesitate to contact us!

No idea how this works - is there a video how to put it together because I don’t know why these clear plastic inserts don’t click into place somewher?

Hello and thanks for ordering our dish! There should be a step by step illustration of this on the insert that was sent in the clear lid of the dish. If it wasn’t there, basically the round part of the insert gets placed onto the round part of the opening on the clear plastic lid. There is a tab on the insert that fits into the small opening on the lid. Then you twist the insert clockwise until it clicks. They call it a bayonet-style connection. If this still isn’t clear please let me know and I will video this for you. Thanks again!

Will this keep out a 14 lb pug?

I can only advise by the experience with my dog who is not a pug but weighed 15 lbs...might be a few pounds heavier these days since I bought this feeder. Once I got this feeder he never ate the cat food again. It made life easier. Now I have to keep the cat out of his dog food. Dogs like to stand & eat and he would have to crouch down to eat the food to get his head inside. There's also clear plastic covers you attach over the openings which makes crouching necessary to stick their head inside of he feeder. My dog had a shoulder/leg injury when I rescued him so uses 3 legs mostly so crouching also may be difficult for him so he doesn't even try to eat the cat food now. I actually am going to order another one of these as my feeder is beat up & taped together. There may be sturdier cat feeders out there but I'm sticking with this one because of it's success keeping my dog from ever eating the cat's food. I put 2 small metal bowls in the feeder for easier cleaning as the feeder doesn't come with removeable bowls. The clear plastic is thin and I had to tape the covers to the lid as the tiny discs holding them in place snapped off very soon. Last wk I dropped a frozen bag of soup on the lid so it's cracked bad but still working with good old scotch tape holding the lid together. I would say take a chance that this feeder would work for you & your Pug.