Batch creating sites on different hosting services

I have a number of domains pointing to a couple of hosting services and I want to use batch create in these domains to create new sites. It seems from the documentation that this is possible. I have 3 main questions. First, I have about 100+ sites split between the the hosting services so what are the main criteria for selecting a host for a multi-site network. Second how long would it normally take to configure the sites from single site to a multi-site network. Third, can I keep the existing themes?

Regards,

Mark Malwah