Ning Profiles

I want to use the Ning export on a large network (well over 10,000 users.) Just a couple of quick questions:

1. When mapping the profile feeds, how will different types of field match? Like on ning, say I use a multi-select box.. if I enter a multi-select box on buddypress, will all those fields switch over perfectly? What if a selection has no match - will a field just contain different text? Just wondering how worried I should be before I start a massive import.

2. I'm using the supporter plugin on my new buddypress site. So there is in a sense, one main blog + community, and then those members have the option of starting their own blogs, right? So do I import the Ning site into the main buddypress omain? OR do I map a new domain using the supporter/wpmu system and then create a little social network/blog as simply the first example blog network in what could be a large series of blog networks all within the main site? Bit confusing I know.... So just wondering if the old Ning site should be imported into the main site or just be the first site to USE the main site as a platform...