Stated another way - you want to guarantee 5Mbps to a specific port (and also limit it to that same 5).
(Just making sure I understand the design)
You need to use mangle table to do connection marking on packets to/from the special port.
Mark all other packets with a default mark
Set up a simple queue that is the master queue and give it the bandwidth of the full amount available to your site.
Set up two more queues with parent queue is the global one.
- one child queue matches marks for the special user and has limit to 5, limit-at to 5 as well
- other child queue matches default mark, but has no limits.
When testing, you can look under ip firewall connections - your connection marks should appear next to connections in this list if the mangle rules are working. You should be able to see all traffic statistics on the various queues as well.
If you make a download from the default policy network and max out your speed, you should see the main queue and the default queue both turn red, but the guarantee queue stay green.
This is a general idea of how to do what you want.
Here is a full manual page for queues:
http://wiki.mikrotik.com/wiki/Manual:Queue
(Note that the configuration example matches on a particular range of IPs and subjects the entire network to a single limit as well, but does not guarantee that bandwidth against users on other interfaces.)
