Thanks for the additional information.
Hopefully, you're only dealing with a temporary set back for the two who are a little more lethargic such as possibly a drug reaction to the Revolution or possibly from anemia from all the fleas that were feeding on them.
The fact that all are now eating and not vomiting is a very good thing, as you no doubt know.
I probably wouldn't be too concerned about their weight loss at this point since there's been so much change in their lives in such a short period of time. But if it continues and/or the two remain lethargic and/or other symptoms develop, then there may be other, potentially, more serious issues going on with them.
Feline Leukemia and Feline Infectious Peritonitis are viral diseases which are frequently seen in kittens this age and for which there are no effective treatments.
Feline Infectious Anemia could have been transmitted by the fleas they had. Unfortunately, antibiotics would be needed to treat them; there aren't any over the counter options.
If significant diarrhea develops and/or they start to vomit again, then Panleukopenia may be the problem. Kittens can survive this disease but aggressive treatment is needed for them to do so.
1. At this point, I'd separate those not gaining weight if at all possible. I know this may not be practical but at least you'll be able to more closely monitor who's doing what in terms of the litterbox and eating.
2. You've already wormed them for roundworms with the Revolution but should you see "sphagetti-like" worms in their stool, I'd worm everyone with Pyrantal Paomate which is readily available at most pet or grain stores. I dose every two weeks for three doses.
3. Continue to remove as many fleas as you can from them if they're still present.
4. I don't know if you're feeding free choice kibble or canned food but I'd probably offer both and make sure there's always food available for them.
Stressed cats/kittens often won't eat so if the food is there for them when you're not around, the shyer one(s) may eat it when they feel safe.
5. Continue to weigh them at the same time every day. Unless you have a really accurate digital scale, there can be daily fluctuations or slightly inaccurate reading; weighing them at the same time may help to minimize this.
In the absence of any blood tests or physical exams, the best you can do for them is to provide adequate nutrition and prevent further harm from internal and external parasites.
I hope this helps and that they will do well under your care. Deb