Also true, so it would have to be closely supervised for a long time
You would not necessarily have to supervise them very long at all. The only time I tried introducing keets to a broody hen (she didn’t have any chicks yet), it was PECK-PECK-PECK and I had 3 dead keets before I had a chance to do anything about it. It is difficult to get a broody hen to adopt chicks under the best of circumstances (maybe 50/50 chances of working). By mixing ages and species you are giving that hen even more reasons to reject the new keets. Your idea is most likely to end up with dead keets before you have a chance to fix the situation.
And all of that merely addresses the broody hen side of the equation. R2elk gave you excellent advice on guineas. Buy at least 20, they are not happy in small groups. And do not raise them with chicks, they tend to become aggressive towards chickens when raised with them as babies.