OK, a few questions...
I've heard that guinea birds make excellent "guard" animals, as they make quite a racket if animals foreign to their area comes by. Is this true?
Yes this is true... Guineas are known as barnyard watchdogs and will typically sound off at anything new/abnormal on your property, not just other animals or predators (could be a trespasser, mailman, UPS or FedEx delivery person, something new in the yard that wasn't there yesterday, a plastic bag flying by in the wind etc). Younger or newer birds tend to be more prone to sounding off at any little thing than seasoned, settled in mature birds do. A lot of people dislike the constant racket Guineas can make (besides the alarm call/sounding off the Hens can make their buck-wheat call non-stop)... so if you have close neighbors, or a low noise tolerance yourself that might be something to consider prior to getting Guineas.
Also, can chicken and guineas co-exist so I'd have a year-round egg supply?
They can... some have mixed flocks with no issues, some have mixed flocks with nothing but issues. It really depends on your flock dynamics and your coop/run set up, breeds of chickens, the amount of space the birds have, the amount of land you have, how much they can free range, etc. Guineas can be very chicken aggressive in some flocks, especially in over crowded coop/pen situations and during the breeding season. Plenty of free ranging time every day is key to a peaceful mixed flock, plus having enough Guineas to keep them occupied as their own flock and less likely to become chicken aggressive helps but isn't always a fail proof solution. IMO, having a broody chicken Hen hatch and raise keets for you and then integrate them into her chicken flock is your best bet, but even that can have it's down falls.
Guinea Hens are seasonal layers, they only lay Spring thru Fall (exactly when varies by climate/temps), so I'm not sure if that equates to a year round egg supply solution for your area/climate or not. Guinea Hens prefer to wander off to very secretive, well hidden places to lay their eggs (like in thistle patches, berry brambles, poison oak bushes etc, lol). They typically will not use nesting boxes in the coop if they have a choice about it. Chances are if they are let out to free range then they will choose to lay their eggs outside somewhere, not in the coop. Nesting Guinea Hens (and their eggs) more often than not get taken by predators if they are not cooped up each night... which means you'll need to hunt down their nests so the eggs can be collected (by you, not the predators). They are sticklers for privacy when laying/nesting so if they see you collecting their eggs, messing with the nest or come back to an empty nest etc they will usually abandon that spot and find a new one to lay. Hunting down Guinea eggs/nests can be a full time job...