Hiya! I'm no expert on guineas (there are a few on here though!) but I do know that it's not recommended to keep rabbits and guinea pigs together. There are a few reasons for this:-
Guineas require vitamin C in their diet and rabbits don't. Rabbits require additional vitamin D. It is possible to get round (feeding lots of veg, making sure they get lots of sunlight etc) but not ideal.
What you were told about injuries is correct - when your male rabbit matures, which won't be long now, he is likely to get a bit randy and try and hump your guinea pig. This will be extremely distressing for the piggie and could cause it serious damage. Sometimes rabbits will get actively aggressive towards piggies, sometimes they won't. But also they can accidentally harm them, for example if they are scared, they could easily catch a piggie with their big rear feet and do it a serious injury.
Finally, both rabbits and guineas are social creatures but they will not understand each other's "language". It is far better to keep them with their own kind, they will be much happier this way. As far as I know, it is fairly easy to keep female guinea pigs together in a pair (I'm sure someone who knows more about piggies will be able to advise you better).
Both the rabbit welfare association and the RSPCA advise against keeping rabbits and guinea pigs together for the above reasons! As to what you do now - ideally, get another girlie pig for your existing one (
www.guineapigrehome.org.uk has details of rescue centres with existing guinea pigs needing homes - you may find that one of them has an ideal piggie and will help you with the introductions). Equally, your bunny would be very happy with another bunny, but he needs to be neutered first - he is nearly old enough for this to be done, and 6 weeks afterwards, once his hormones have dropped off, he will be safe to introduce to a girlie bun. Again,
www.rabbitrehome.org.uk has details of lots of rabbit rescues and many of those will help you find a suitable partner and introduce them for you, as it is important to do it in a certain way, otherwise there could be serious fights!
Hope that helps a bit, let us know what you decide to do