Ventrilo is a voice communication program - it is completely a separate server from CS:S and any would-be plugins (ZH).
If you wanted a server with Zombie Horde plugin it would be a Counter-Strike: Source server. The # of player slots is not fixed at 16, and you can adjust it on our ordering page here:
http://rentals.nuclearfallout.net/rent. ... tent=order
Also, you are limited to running 4 bots and these are included in the # of player slots that you buy. For instance, if you bought a 20 slot CS:S server you could run 4 bots and 16 human players.
If you also wanted a ventrilo server you could order it from the order page in a package deal, under the "Choose whether you'd like additional servers included for a small additional fee" section.
I hope this clears things up for you #1. Let us know if you were thinking of something else..