I personally would not keep tortoises together that had a great size difference.
Adults I wouldn't worry about, But young I try to keep within an inch (length) of one another. Hatchlings I would only keep with hatchlings.
Larger torts tend to crowd smaller ones & hog the food.
I agree with t-mclellan, it's not a good idea to keep tortoises together that are greatly different in size. Simply because the larger tortoises don't acknowledge the existence of smaller ones. The smaller ones would be at a disadvantage and will get trampled, knocked around and definitely cannot compete for food.
I got 2 hatchlings at the same time 3 years ago. They were supposed to be the same sub-species (they were both Greek), but unfortunately that wasn't the case and they quickly became very different in size. Our bigger one would bully our little one horribly - ram her against the wall, as well as mentally bullying when it came to food and resources. Maybe our bigger male was more of a bully than your average tortoise, but I know once we separated them they were both much, much happier. I definitely wouldn't keep 2 tortoises of different sizes together ever again! Here is a pic of their size difference before we officially separated them.
They both look to be Ibera tortoises Testudo gracea ibera. It is not unusual for tortoises of the same age and even from the same clutch to grow at very different rates. Probably a natural form of species preservation.
Male Ibera can be very aggressive as they approach and reach sexual maturity. A dominant male will often terrorize anything smaller than itself, male or female. I actually have two equal size females that are constantly going at each other.
No. Do not do it. Don't. I have two tortoises only TEN months apart and I cannot even imagine what the older girl would do to the smaller one. You are setting that baby up to fail, IMO.
This would completely stress the smaller tortoise out... also nobody seems to have mentioned quarantine... but i have just skimmed through...
Any new additions (of a similar size) would need another set up for a quarantine period before introduction...
Also you could end up with another male... bring on the fighting... then would have to seperate anyway.... best bet would be to either stay happy with what you have... or hunt for a FEW females.... but then they would also need a break from a randy male... so again seperation...