I'd imagine that if you use the recommended dilutions you'll be OK. You want to keep the cactus right? I believe they have to apply it undiluted to the cut surface of prickly pear when trying to eradicate that.

I have a BIG mound of hen and chickens and in summer all of a sudden it had darned kikuyu all through it - ALL through it.
I thought oh well I will take a couple of pieces of the H and C and if Roundup kills it along with the kikuyu I will just replant.
The H and C was just FINE thanks... and the kikuyu karked it beautifully to a dead easily removed brown mess, so I would say it owuld be fine.

Be careful with Roundup, especially on cacti. You may not notice any damage for months. In fact your plant may still grow and flower beautifully. But then all of a sudden it can turn yellow and black inside and drop dead. This is because cacti have such SLOW metabolism that if any of the chemical enters the tissues, it will take a very long time before it reaches the roots. Roundup is only biodegradable out in the open air, such as on the soil, where it is broken down naturally. If even the tiniest amount gets inside the plant tissue however, it can remain active for much longer.
I've heard of people saying that Roundup is safe on orchids as well. This is nonsense. It still affects them, just a lot more slowly than with other plants. I once knew a nurseryman who would spray Roundup all over his orchids to remove weeds from the pots before selling them. I only found out that he was doing this after I had purchased dozens of plants off him. Every plant I had ever purchased from him had died within 12 months. Roundup can actually produce an initial growth spurt and cause profuse flowering in some plants, after which they go backwards and eventually die.