I'm trying to find similar answers myself. I've spoken to Skype support a couple of times and they insist that any Skype connection is limited to 2 concurrent channels. But Skype for SIP has a (i think) 300 channel limit (depending on what you purchase), so I'm not sure if the support were unaware of the proper answer, or if S4SIP is treated different.
Regarding the incoming calls, it seems they're free. I have successfully made incoming test calls to my SkypeIn number routed to my SFA using a zero balance account.
Interested if you have a more accurate answer to the first part tho.