Overweight and Underweight

Ok so Joey and Keiko are both getting chunky... Joey more so.
Kahlua needs to gain weight. She is 3 years old and weighed it at 4lbs when we took her to the vet last. She is the size of most 7 month old kittens. She has always been itty bitty. But we would like for her to get up to 5 lbs just to make her spay a bit easier. I've heard that some vets wont spay a cat under 5lbs. And ours would just like her to gain a bit more. She is perfectly healthy, just very energetic! She had gotten up to 5lbs but then Keiko moved in with us and now she is NON STOP playing!
Is there a food that we can feed all of them? It gets very very hard to lock Kahlua in a room to feed her! She is a grazer (actually all 3 are) and she hates being locked in and will scream, claw and the doors, and half the time open the doors.
We leave dry food out so they will all graze but we only put about half a cup and we refill it once a day. Is that too much?
Any ideas?! Please?

some indoor foods may work...

I would suggest adding wet food as it aids in loss and maintence

has the vet said they are over and under??

That's odd that they wouldn't spay her under 5 lbs. I had my rabbit spayed and she only weighs about 2.5 lbs!

well part of that is also my not wanting to because she's so small. My cousins kitten died after complications and they think it was size. But thats was a few years back. and nothing was ever said for sure.

Joey refuses to eat wet food. We are still trying to find one kind that he likes. But we are running out of options...

The vet told us that Kahlua needed to gain a bit more weight, Joey and Keiko are both just getting hefty.