As far as incompatibilities are concerned, there might be on in form of government. Let me quote the (unused since they’re always friendly to us) taunts for Hjorden ships on comms.
<hullRace ID="9" name="Hjorden" keys="friendly"> <taunt immunity="is an Artist" text="You should fire your Ship designer!"/> <taunt immunity="is a Member of the Aristocracy" text="You should go back to your hovel pleb!"/> <taunt immunity="is an OCD Cleaner" text="That ship smells so bad I can smell it through the vacuum of space!"/>
Judging by these taunts and especially the Aristocracy one it is very likely the Hjorden government is some form of feudal or absolute monarchy which in itself is a stark contrast to the democratic vibe of the USFP. Also depending on how decentralized the Hjorden government actually is it may very well have sovereign components within that have issues with their primary leader handing over the keys to an even more far away government ( as far as I know Earth is pretty far from Hjorden).
2. We’ve never really defined the Hjorden homeworld or any of their colonies and we’ll have to figure that out here.
3/4. The Hjorden are known to use a few classes of traditional(mechanically) constructed ships for policing their space and carrying around loads. The first ship and their prime (and only known) war vessel is the Hjorden Cruiser armed with reasonable defenses and beamers akin to those found on TSN Destroyers. Besides this there are also Hjorden cruise liners and transports which carry passengers and cargo between Hjorden bases. Their forces are definitely geared towards their somewhat neutral and isolationist stance and it shows in their innate defensiveness and general weakness (not to mention lack of ship variety as of this time) on the offense.
Ultimately, if diplomacy succeeds the Hjorden could become a useful ally to the USFP given the current invasion of Cerberus and increased hostilities along the Promethean Border Region as a whole. Such a military alliance would be temporary but would be beneficial to both parties. The Hjorden aren’t particularly fans of whoever tries to dislodge them, like the Pirates.