I realize it's been a couple of months since you posted your question and I'm hoping you already have answers to your questions.* If not I will try and help.
You asked:* "Is a clay pot the best to re-plant in?"
I think so as UNGLAZED clay pots 'breathe' and allow the moisture to evaporate.* Since cactus don't like their roots wet, I find that best.
You asked:* "Do I still follow the suggestions of going with a pot only 2 inches bigger than the current although these cactii haven't been replanted in about 10 years?"
Unless you see they are stressed from being rootbound, it would probably be best to go 2" to 3" larger.* The potential for overwatering in a larger pot is too risky as the soil would stay wet longer.* If they are severely rootbound, as I suspect they might be, consider teasing out the outermost roots so they don't continue to circle.* You can measure the width of the rootball and use a pot 2" to 3" larger then the rootball.
You asked:* "Do I leave the root clump the way it is or separate when re-planting?"
If they are happy together and their tops are not crowding each other, they can probably be left together.* Without pics it's hard to say.* This site explains how to remove and pot up the offshoots.
You asked:* "Is 50% sand and 50% mulch the best mix for this situation?"
Not knowing which cactus you have that sounds like a good mix.* You can also purchase potting soil made especially for cactus.* Check to see if it contains a slow release fertilizer as you could burn the roots if it does and you add additional fertilizer.
Maybe this site will be helpful.
Hope that helps,
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.