Importing best practices and questions

I have a list of 800 accounts in csv format. Most of the accounts are leads, some of the accounts are customers. As a general rule of thumb, where do you recommend a list like this be imported to?

If all are imported to leads, those that are customers could be converted to accounts. Do you recommend only one import location per list? Or, should I separate the leads from the customers and import 2 lists separately, one to leads and one to accounts?

We just started with the free trial version and while experimenting with importing and custom fields, I have already reached my limit of 25 customized fields. Can you you extend an additional 25?

While experimenting with importing to different locations and custom fields, there have been imports where only a few records are skipped and others where 100's were skipped. What causes a record to skip?

Thank you, Steffen Sheerin