We have a neutered male and an unspayed female (working on that one though!) that have been together for almost 2 years now- male we've had since he was a baby, he was 3 when we got the female from a friend (they didn't want her any more), she was about a year old.
I think we've been lucky as we haven't had a single problem with them, it took them a week or so to get used to each other, but now they won't be apart and snuggle up all the time.
The only advice I can give for convincing your DH is telling him rabbits live in a colony in the wild, and a single rabbit is quite often an unhappy one. Rabbits and guinea pigs really aren't suited to each other, different dietary needs and there is always a risk the rabbit can harm the guinea pig, by accident as well as on purpose.
I convinced my DH to let me have our female by stressing how much happier our male would be with her, and he definitely is. He's lost weight (he was quite fat before), is much more active, and in turn she is a lot more friendly towards us (part of the reason our friends didn't want her any more was that she was "nasty"), she is also half blind, and having him there really makes her feel secure.
Sorry I've written so much! My rabbits are very close to my heart, it's such a good thing you're trying to do for yours
I found this link, but there are loads more out there as well.
Good luck!