I'm not sure if you got your animals yet. We have 16 'pacas and a small herd of beef cows.
I'm not familiar with the zebus breed but you can mix the pacas with most cows easily. We usually put the orphan calves with our alpacas because of the needed attention. You will need fences that will hold the cow but if you are raising the pacas for the fiber, do not use barbed wire.
You can easily pasture 3 or 4 alpacas on an acre of pasture and probably still have to mow once or twice a year as an alpaca eats much less than a cow. On the other hand, the cow could easily eat an acre of pasture pretty quickly, depending on the quality of the forage in the pasture. The nutritional requirements are different so you will probably need to supplement the cow and keep the alpacas from eating it.
What I would do is, divide the pasture at least in half and maybe thirds. You can rotate the pastures and use them to separate your animals if needed. I'd offer free choice 2nd or 3rd cut grass hay to all but if the cow gets lazy and just eats the hay, you'll need to control it more. I'd rotate the cow through the pastures and if you get to the point where you are endangering your pasture, get rid of the cow. The pastures are what we take care of first. If the load gets too high on them, we lighten it in some way.
Hope it helps. You will love the alpacas.