Hi, if you were irresponsible you wouldn't be asking the q's
Guins should not be mixed with animals of other varieties, particularly birds and including ducks.
Yes you can treat injuries, would most likely be abscesses and need lancing (sometimes every day, a vet would do this) then flushing. I flush with Baytril (an antibiotic) I also give antibiotics orally- thats a lot of toxins and you don't want them in the bloodstream. Flushing would be done at least twice daily then after a while you may be able to cut back to once. There are no injections for guineas for prevention.
Skin problems: Guineas exposed to damp conditions (they have not evolved to cope with our humid climate) are more susceptible to fungal probs. In addition to this if you're not handling the pigs you won't pick this up early- hence the need for handling or at least definitely being able to check them over, can't do that if they are always under a hutch
Guineas that are exposed to the elements may be more prone to having mites 'kick in'. All guineas carry mites and when the guin is stressed (fighting off cold/illness) then they will take hold, again you will not notice till quite late because they are not handled so more treatments (Gorgeous Guineas Melts and Ivermectin) is needed.
Parasites: Guineas will be more prone to picking up parasites from overgrazed grass as the run is statutory and they are in it all the time- no rotation, I would have a shed to let the grass recover and the guineas still have the exercise but you don't seem keen?
Vitamin D: They'll get lots of this, one benefit of being kept this way
Guineas vet bills can run into hundreds. An op is not much change from £100 including x-rays. Or you can have them put to sleep, thats cheaper. I thought they were going to fend for themselves? Bladderstones are common in guineas and after the op its advisable to give water (40ml) by syringe daily to prevent more from forming. Re health issues read the Health and Illness board