It's a possibility. I have not kept Guineas with duck so I do not know how that usually works out. I know most ducks aren't all that aggressive tho, so they could be facing Guinea aggression issues later when the Guineas mature.
The reason I suggested getting more is because Guineas need plenty of their own kind to keep them preoccupied/focused on being a flock of their own and mainly picking on each other, especially once they start maturing and their hormones start kicking in. You may not see any aggression from them this season, but starting next Spring it's highly probable.. the males (if you end up with any) tend to lose their minds during the breeding season. Once the hormones kick in they become possessive, territorial and make it very clear to all other birds where they stand in the pecking order. The Alpha male Guineas usually the initial culprits for starting the aggression problems, but the rest of the flock can/will join in.
If the aggression gets too bad and your ducks end up with wounds it can lead to cannibalism, or worse... Guineas sometimes target out and kill other poultry (for reasons that only make sense to themselves). If your ducks have a good sized pond, at least they will have some place to go to get away from the Guineas.
I'd be sure to feed the keets high protein Turkey/Pheasant/Game bird starter feed at least until 6 wks old, and avoid over crowding while they are growing up and once you integrate everybody. Guineas need a lot of elbow room in the coop and the run... and as much free range time as possible once they are old enough should help.
If I were to get more guineas how many should I get? We are putting them in a pretty good sized area and along with the 10 ducks they will be living with two goats (who don't take crap from anyone! they head butt the dogs if they get to close when we walk them around outside of their pen) Thanks so much for sharing your knowledge! Much appreciated!
Flocks of 10 Guineas or more do better than smaller flocks, but if you don't have a lot of land for them then 6 would at least be better than 3. Hopefully you end up with more Hens than males, because that also helps keep the males occupied.
I free range my Guineas in my goat pastures (and with my horses and my donkey) and usually everybody minds their own business, but I have seen some extra cocky Guineas attack the goats before (and all my goats have horns, lol). Nothing major, and it doesn't happen a lot, but it just goes to show you what I mean about the male Guineas losing their minds once the hormones kick in, lol.
Also, if the enclosure you are keeping the Guineas in isn't covered, they will eventually fly out (and more than likely forget how to fly back in).
Having one big happy mixed flock would of course be ideal, but you may want to have separate coop and enclosure for the Guineas just in case they do become a problem. I keep a few Peafowl with one flock of my Guineas, and a few Turkeys with 2 other flocks of my Guineas (all of my flocks free range in separate areas/pastures)... the individual mixed flocks get along for the most part without any issues, but the Peafowl and Turkeys all are big enough to put the Guineas in their place if they need to.