1. sure. you can use one for the outside interface, and put the other 4 in a global pool statement - I have that exact setup for a client.
2. global ip's need to be legitimate for the outside world to connect to. you can either forward complete ips to servers (i.e., outside.ip.address.5 gets forwarded to inside.ip.address.200), or just ports (so you could forward outside.ip.address.6's tcp port 80 to inside host .3:80, and outside.ip.address.6's tcp port 25 to inside host .4:25.
so, you need to determine how you want to use your ip addresses and forward ports, and then write an access list that you bind to the outside interface that allows access to the services you want