I'm moving my hosting out of my house and to a colo facility (yay!) I'll be using H-Sphere to manage the servers, and all servers (for now) will be running CentOS 3.4.

Now, 2 questions: Servers are coming in one at a time, and I'm anxious to get things up and start migrating accounts. Is there a problem starting with a one-server install and growing that to 3-5 over the next month? Would I be well advised to do them all at once, or is this one of those "an hour or so of down-time" kind of issues?
up2date and yum look like great management tools, but I'm concerned that H-Sphere uses its own RPMs. Is there going to be any conflict between yum/up2date and H-Sphere? It's my preferred way to keep up with patches...Thanks. I know these are simple questions, but I'm a little frazzled right now and could use a comment or two from those more experienced. :)

There wont be any problem adding new servers in one or two months with HSphere. you do not need to take any of the existing servers down.

If you plan on using yum or update, there is a guide at http://www.psoft.net/HSdocumentation with details of what packages you cannot update using it.

If you plan expanding your cluster over time be sure to put each service (mail, mysql, web, etc) on their own IP (1 IP/1 service). This allows you to move the service to another server without running into dns issues once you have accounts in your cp.