networking - NAT/PAT port and address selection

I am currently having some issues with TCP port reused too often (under 2 minutes) for a source NAT and I was wandering what is the algorithm used to select the IP and port for a SNAT.

Here is some context: I have an entire network (10.0.0.0/8, tons of users) accessing internet through one firewal (it happens to be a Fortigate).
We have a public address pool (let's says 1.2.3.0/27) to NAT all these clients IP addresses.

How will my router/firewall decide what IP address to NAT to and what source port to use?

Is this vendor specific? Do you know of any implementation that you could explain to me?

I hope my questions are clear :)

Regards,

Pierre