They shouldn't fight much at all as the wethers will quickly recognise the older entire tup as boss. The wethers might test the waters a little, and the tup will have to put them in their places a couple of times, but I don't foresee a problem, unless you have another entire tup.
We would keep the new animal in quarantine for 3 to 4 weeks anyway, before allowing him to mix with the rest of the flock, or put him in with the ewes.
The small pen business is for putting entire tups together, and then the size of the pen is such that they can't have a go at each other, including thumping with horns, so tight, but with enough room to lie down. Their water is best in a bucket of the type which is hooked onto the pen or it will be spilled. We would leave them in together like that at least overnight, then put out some food when they are released into the field to distract them. This can be enough to prevent immediate fights, but they will at some point fight to establish the pecking order. Occasionally a tup is injured, but without letting them settle their differences they will never settle together. They will fight far more near tupping time than at quieter times of year.