Borg Mixed Tech Vessels
Gains the Adapting Disruptor Array passive ability which deals more damage to non-Borg vessels. The class has one less offense and two more defense than the Helev Leahval. For Helev, this ship is not really worth it because not only will you will still have to research its special abilities, but the supply costs are double the normal Leahval.
Gains the aptly named Close Distance Transwarp Drive, which consumes a significant amount of special energy (when refitted you can only use this ability twice before needing to recharge special energy). This is absolutely not worth it for either avatar because you should always be using the special abilities of the Rhienn (Vector Torpedo or Multi-Targetting Disruptor) and thus you can never use the extremely short-range transwarp. Likewise, because it is mixed-tech you will not get any avatar bonuses and yet it will be more expensive and less useful.
Generix Frigate Refit
This class gains two more defense and one less offense. The passive ability Graviton Energy Source is added which grants a 50% increase in the shield regeneration rate. The ship also counts as a station for damage calculation purposes. Thus, torpedoes always hit this ship and pulse weapons will deal 40% less damage to it. Although this vessel is decently powerful when used against pulse wielding opponents note that it will take more damage from specific artillery units and it will always be hit by torpedo weapons. As this Generix is built already refitted, in the strategies that you would normally build Frigate Refits, you can use this vessel instead to supplement your Generix Support Refits and save yourself the research efforts.
Federation Mixed Tech Vessels
Generix Frigate Refit
This vessel gets less offense but higher defense as well as the special ability Critical Shot. This ability disables an enemy ship’s weapons for ten seconds. Since normally a Frigate gets no special abilities, this is a nice way to get some additional use out of Generix Frigate Refits. This ship should be built for both avatars (especially when facing against the Borg where it should be micromanaged to disable high priority targets such as Diamonds and Cubes). When playing the Helev avatar you should only build a few of these vessels and micromanage them, but with Mijural this ship should probably completely replace using and upgrading your own Generix Frigate refits to save on research costs and to give you a great deal more early game firepower (as well as to allow your normal Staryards to continue building Generix Support Refits).
This class gets burstfire torpedoes and one less offense, but two more defense (and system) than Helev’s Leahval. The torpedoes make it much better at staying useful late game against battleships and stations. With Helev this ship is not worth it due to its extreme supply costs, but it must be noted that this vessel does much more damage than the normal Leahval's weaponry (especially when under the effects of Energy Reeling).
Talon Repair Refit
Although this ship has very weak stats, it has the same repair weapon as the Federation Newton vessel (and is also limited to three ships, sadly). Consequently if you can keep it out of battle, it will be able to repair the systems and hull of any ships that are stranded, as well as any stations. Build these only if you have extra funds because of their outrageous costs.
Dominion Mixed Tech Vessels
This expensive class gets a sizeable boost in offense and a slightly larger blast radius. Consequently you need less to do more damage in the field which is always good. As these ships are generally not used in mainstream combat, only build a mixed-tech facility if you are planning on building the other available ships as well (otherwise the costs are not worth it).
This vessel gets the Cascade Feedback special ability in lieu of Energy Reeling. This ability damages enemy units based on the amount of special abilities they have. Consequently this unit is rather powerful against your opponent’s support vessels and thus quite effective against factions that rely heavily on support units (the Federation for instance). Unusually, this ship is actually cheaper than the one available from the Staryard. For both avatars this should supplement your normal fleet when facing the above situations (although energy Reeling must still be researched to gain access to Cascade Feedback).
Rhienn Torpedo Refit
The offensive value has been boosted by one although the defense has been decreased by one as well. The class gains the Tetryon Precharge passive ability which enables it to do more damage to stations. Likewise the amount of special energy reserves has been increased which allows you to fire several more Vector Torpedoes before running out of special energy. For the Helev avatar this ship is not worth it due to the excessive costs and the fact that it comes so late game.
Klingon Mixed Tech Vessels
This unrefitted ship has one less offense then the Helev avatar and yet is oddly more expensive. Obviously not a good choice for either avatar (it will only give plus two offense for Mijural, which is still not worth the extra hundred dilithium you will be spending).
Rhienn Pulse Disruptor Refit
This ship comes equipped with a 4x Pulse Disruptor and the Graviton Marker (which will slow down enemy vessels until they are repaired), but does not have a very high offense (less than both Helev refits and the same stats as the Disruptor Refit for Mijural). Likewise it can only fire from the front and from the sides. Combined with its medium range weaponry, it is only good for killing destroyers and could potentially be useful for support due to the Graviton Mines if it were not so weak defensively. Building a few of these later game and using their mines before setting ambushes is potentially powerful (especially for allowing your units to catch up to long range and moderately fast ships like Excelsior II’s).
This ship comes equipped with the Concave Plasma Conduits special ability which gives the class plus 25 in offense for 20 seconds at the cost of one supply per usage. Note that this will temporarily disable cloaking. It also is equipped with the Fast Tracking Computers passive, which makes it excellent at doing damage to short range units, but taking more damage from long range units.
Its particle weapons do increased damage to vessels with a defense of 20 and below, just like the Generix Frigate Refit (and is roughly the same cost too). The Concave Plasma Conduits ability gives it a sizeable offensive increase over the Generix Frigate Refit for a fair amount of time, and although the ship's defense is considerably weaker (and its normal offense is slightly weaker than the Helev refit) the vessel is now quite powerful. The supply costs for special weapon usage are negligible (equivalent to only single digit values of dilithium and tritanium). For Mijural and Helev this is a useful ship.