Poseidon is not bound to a special hardware. As long as there is a lowlevel hardware driver for it, Poseidon supports it. There are working drivers for Highway, Subway, Pegasos, XXX (top secret

) and Spider. The Thylacine driver still has some problems, concerning mass storage devices, but otherwise seems to work fine. The problem is that most Thylacine users don't give enough detailed feedback for Robert to fix it.
Trident is the graphical user interface and prefs program for Poseidon. CrossDOS does also work with Poseidon's mass storage class for automounting, but I really encourage to use fat95 (the UMSD mountlist is part of the Poseidon package).
As I said, I don't think Sirion (the included software for the Thylacine) currently has mass storage support (at least, not publically available -- I'm not saying that there isn't such a working driver in beta stage). Poseidon on the other hand, doesn't currently have a class driver for Ethernet cards as Sirion has.
You can download the demo version of Poseidon from Aminet. It can be installed at the same time as the Sirius USB Stack, but it cannot be /run/ at the same time. You will need to contact Robert to send you the latest HCD for Poseidon, as he had removed it from his website. The demo version is time restricted and will stop working after some (short) time.