No problem.

Yes, it is necessary.

If you're running a Slimcoin node at home and want it to be a listening client, then you'll need to ensure that your router supports UPNP and that the router is configured to allow incoming connections to the port and ipaddress of your node. UPNP allows your local host to connect via the default Slimcoin port, then you can advertise your ip address as a listening node for addnode inclusion. Do bear in mind that running a listening node at home will impact your upload usage if the size of the network is significant, as other nodes access your node for data which your node then uploads. Overloading your upload channel can nuke your download channel as the upload channel carries sync information for both up- and down-stream and a maxed-out upload channel can bring your download channel to a halt.

Of course "up" and "down" are entirely perceptual. I once used the term "download from" in conversation with a network hardware engineering friend of mine. He cocked an eyebrow and said that he always thought of the net as below him, not above him, so he downloaded to the net and uploaded from it. I thought that was an excellent perception and have followed it ever since.

Cheers

Graham