Hi Chris,
yes you can do all that.
Use SNAT to translate outgoing connections Source address using one, defined as an alias interface.
Use DNAT to translate incoming connections Destination unsing a private ip defined in definition networks.
Use Masquerading on an interface for the rest of the provate ips tp go out.
thats right, you can not masq on alias interfaces, use SNAT instead.
Don't forget to set packet filter rules.
kind regards
polluxxx