If the zombies are coming and you need to use a home-made silencer on a rifle, then it would be best to look for a rifle that uses handgun or rimfire ammo. I’m not sure, though, if it is better to have multiple guns that use the same ammo, or to have multiple guns that use different ammo.
If you have a really good supply of ammo at a home base, it would be nice to be able to swap ammo between the guns in the field or share ammo between team mates.
On the other hand, It would really suck if all of your guns use 9x19mm Parabellum ammo, but you’re in a town where all of the 9m Para has already been looted (but there’s several boxes of .38 Special).
If you’re scavenging ammo, then it would be best to have guns in as many calibers as possible. Ideally, your group will have enough guns that you can use any kind of ammo you find.