You can safely do three static IPs on that vultr server. I've been running 3 dashd servers on one vultr instance for months without issue. You will need to delete the debug.log file periodically because they'll eat up your disk space.
I needed to use Tao's guide to make my first MN, so would I be right in thinking that I would need to duplicate the instructions after the firewall has been made, starting with a new user in putty
I hope a lot of people don't do this, because the whole point of running a masternode is to make the network stronger. Each 1000 dash pays more than enough to cover a full VPS. I therefore run VPSs in separate countries. I have one node in London, and another in Switzerland. Running a whole bunch of masternodes on one server defeats the purpose.