We get snakes a lot – the pythons are threat at night so we put our poultry in small houses made of pallets around the inside of which we nail chicken netting making complete coverage against most snakes. Mambas are slightly more versatile and the younger snakes can penetrate chicken netting [we lost a half-mature gosling at one point] – the answer seems to be goats which mambas seem to avoid. We housed our milk goats next to the poultry houses and have not had any issues since. Mambas can be quite bold – recently I have had a couple pass beneath me as I sit out in the evening.
In addition to your idea of chickens and ducks, I suggest geese and guineas - they are good at warning of snake attacks. Guineas do not mix democratically with chickens, however, and so much depends on your breed of chicken.
As to monkeys I guess it may depend on which type you have. Vervets can be very bold and habituate to humans, hence the need for catapaults to remove the habituation. In the maize-growing season, all in our neighbourhood play the game of ‘Pass the monkeys’ since when we drive them off, they become someone else’s problem. The early plantings of maize this year having mostly failed, we find it easy enough to drive the monkeys off onto neighbouring plots. This year, there are few monkey-feeding tourists around at the coastal lodges and so the monkeys are hungrier – the problem is that their natural habitat has been lost over the years and so they need new food sources. If we had forests instead of endless sisal plantations the monkeys would be less of a problem.
[Habituation to humans can be a real problem - one of my elderly neighbours fought a battle with pots and pans against a blue-balled male vervet in his kitchen the other day. The neighbour has had a triple by-pass surgery and so the battle was not advisable although the old man finally prevailed with a frying pan and subsequently – for him, not the monkey - a large glass of whiskey.]