If you get adult guineas, you need to be prepared to keep them locked in a coop for a minimum of 2 weeks, preferably 4-6, for them to adjust to their new home. Otherwise, they are quite likely to leave and head back to their old home. For this reason, most people find it easiest to start with keets. Also, this is the time of year when keets are hatching, so they should be fairly easy for you to find.
I don't think you should have any troubles with guineas and ducks, as long as the guineas are not required to live closely with the ducks. Guineas don't like water as much as ducks do. I don't have ducks though and cannot speak personally to this issue.
For your happiness raising guineas, I recommend:
1) Get a minimum of 10 birds. Guineas are flock animals and they are only happy when they can interact with many others of their species. Also, the number of acres you describe, you will want at least that many if you are going to notice much for bug control.
2) Raise the guineas separately from the chickens. I don't expect you will have trouble with this, since your chickens are already older, but I am mentioning it because it is such a common complaint. Keets that are raised with chicks often turn into guineas that bully chickens.
3) Move your guineas to their adult housing, whatever that is, as soon as you can reasonably do so. Guineas hate change and adapt slowly, so it will take them awhile to get used to the new housing. I think a commonly recommended age is around 6-8 weeks, but honestly you can do it at any age, as long as you are prepared to either lock them in the new housing for a couple weeks or every night, chase them into the housing. I've done both and definitely recommend locking them in, chasing them in every night took MONTHS, and their new coop was in the exact same building, just the next room over.
Once your guineas are adults, you may or may not be able to keep them in the same coop with your chickens. My chickens don't particularly like or enjoy when my guineas decide to roost in the chicken coop. Your experience may vary.