Really depends on the geckos themselves. There could be no difference and fighting could still occur. I wouldn't go over 5g either way though. Just remember that they all grow at difference rates too so it's possible for one to overtake the other.
I wouldn't try to make the smaller gecko 'catch up' as it then means you're making it grow quicker than it naturally would. I would just give it a cgd with live 2/3 times a week and allow it to grow naturally. They tend to have growth spurts anyway.
Using the bigger viv might work better like you said. Just make sure there are plenty on hiding places so they can escape each other if they wish. Also 2/3 feeding areas too prevent fighting over food.
I didn't mean do anything to super speed up the growing process, just keep it a few degrees warmer than the other and a couple more crickets each week more than the other one.. Just to take a couple of grams off the wight difference!
Not bake it in the oven for 3 months whilst force feeding it like a foie gras goose!haha
I have two females in together at the moment, one at 28g on last wiegh, the other at 37g. The smaller is the more dominant of the two, the larger generally sits about and eats any live food from the tongs. The smaller enjoys hunting hers. They make a brilliant pair, and adding the smaller went brilliantly after the largers sister died.
When my 17g female grows up a bit more she will be moved in with the former two IF the introduction goes well. I think we may have a few issues with her and the smaller female, as they are both dominant.
With the smaller geckos I have them split in to two groups. One consisting of a 12g a 17g and a 16g. The other group are 9g, 4g and 5g. The 9g is in with them as she doesn't get on with the 16g in the larger group, which is where I would really like her to live. She gets on brilliantly with the smaller two, so this is where she will be staying for now.
Some geckos get on, others don't. My two tiny ones had to be split because they fought, but other than those I haven't had many issues.
The main concept with putting similar sized geckos together is simply that the bigger one, is stronger and heavier so can therefor do more damage to the smaller one if a scuffle did break out. And of course babies with adults may very likely end up a messy outcome.
Personally I wouldn't put animals together that are either 15% larger or smaller than the other. But at the end of the day any size could fight. One of my smaller girls used to cause a lot of trouble in her trio, and she was the smallest so we had to separate her.
But a good size guide in my opinion is 15% each way. So if you have a gecko that is 35g, then you are looking at either 5g smaller or bigger.
I originally thought they would too to be honest, but when I had a fight break out between my larger babies I had to move one, I knew she got on with the smaller babies as she was a clutchmate to one and had just outgrown them.
A while back as mentioned two youngsters had to be seperated, the smaller bit the bigger and made it drop it's tail. It's the first time I've had one drop so it was a shock.
One of my males will only go near frog bum cresties, he is a tail nipper.
I would never put any small enough to be viewed as a snack with each other.