Do you see clients with piercings, specially cock rings and studs?
And have you ever had a pierced client rip a condom?

I've got a couple of clients who wear cock rings which to the base of their shaft. Never had any problems except one had one which sat just under the head of his cock which made me wary if it would split the condom, there was also a stud. It was also uncomfortable for me in certain positions so had to ask him to take it off halfway.

I only had one client with a cock ring but no problems with the condom or the sex

I've only really ever had one or two with cock rings and it's never been an issue.

I once saw a guy where he had at least 3-4 piercings including under skin body mods all just on his dick, could barely see the thing haha! and they were also very thick as well. I used my king size condom (as he was girthy and big anyway) and it was no problem.

Whenever I worry about condoms I always think back to images where someone has put their entire leg in one without it breaking and think to myself 'its probably going to be fine' 

As long as there is lube, the condom is in date, put on properly, I think you'll always be good to go.

Right length width-wise so its comfortable helps too. The guys I have seen when I knew I would need a king size for it to go on properly even if they are not big length-wise and their face lights up lol, the same guys that say 'condoms are so uncomfortable it hurts' are literally cutting off dick circulation half the time with regular condoms and thats why they don't like them too Either way a regular condom will usually still work on girthier guys but because of the width I think there may be more of a chance of it breaking too so its not worth the risk.