Would like to be able to read this book again, and see if there were sequels.
I read it around 2000-ish, no idea on the published date. Unfortunately the names of people/races/places I cannot recall, but the story line I remember fairly well. Its kind of a long post, but tried to have enough points in for someone to recognize it.
The earth was battling to support humanity, so they created a number of deep space ships and sent them off in different directions to find new planets to in habit. Some were found by an Alien race who assisted them in finding new planets, while a few ships were never heard from again.
Years later all the colonies were able to connect again, and humanity populates large amounts of space, with Travel between being the norm. There was only one other alien race known about, which were on friendly trade terms with humans.
Most of the humans remained the same, but one of the planets had unique plants species which the humans learned they could bound with and be altered. Not everyone on the planet did it, you could choose it and be trained. These people then took on a green skin, and were able to communicate with each other telepathically across any distance if they were touching a tree from their world. They would often be part of diplomatic groups or could be hired for instant communication.
Humans were governed by a King, which is just a figurehead while a group actually controls everything from behind the scenes. The public do not know, they think the King makes the major decisions. They do not have seem to have total control of every planet directly, there are government systems in place for each area and planet.
The alien species people are breed for specific tasks roles which change them physically. Example is the warriors are larger and naturally armored. Their leader/king is physically linked to his people though they do not communicate with each other telepathically - example when one of their ships explode he can feel all their deaths happen. The people do not like being far away from the their own, even if they are in a large group, like they can feel they are away from the link.
When a to-be-King is young he is encouraged to breed with as many of their different people to keep strong bloodlines. Once he comes of age to ruin he is castrated and never breeds again.
Interstellar space ships are powered by gases harvested from gas giant planets and processed in a fuel. One of the original spaceship groups chose to live only in spaceships, and they are the main providers of the fuel to both races. The Alien race were the original harvesters, but happily gave up the role and equipment to these humans as they did not like being far from their own race.
Some parts from around the story
The human king's successor - The current king figure head was getting tired of the role, so the rulers set to create his replacement. They chose a boy who was very similar in looks, and set out to murder his family by staging a fire in the apartment block. They then came in to "help" him and offer him the role. The people are then presented him as the soon to be new king who had been "hidden" from the public until he came of age. They then trained him in his role as the figure head. The boy does eventually start to suspect somethings wrong, and finds reports about firefighters being unable to get to the floor his family lived on because the doors looked like they were welded shut. He decides to play along and learn all he can about the rulers.
Alien prince - The prince features a lot in the story, the main line is he falls for a human who's from one of the "plant people" who was hired for communication. She falls pregnant, but before she can tell him he leaves for a diplomatic mission, during which the Alien King fakes her death in an accident (while typing this I'm realizing a trend with this author :) as he has some plan with baby as his "only" option to help his people in the war (see next point).
Spoiler Note - If the book sounds interesting at this point and you want to read it (and hopefully someone knows the book at this point) I suggest not reading the below main plot spoiler.
The start of the war
Humans had been working on a project that if I remember correctly was based on a concept of tech from some long gone race. It was be able to ignite a gas giant and turn it into a sun. There was pretty much no use for it other than a "shock and awe" to show how much humans could actually do. Unknown to the races (except possibly the Alien king as there was hints that he knew more about it) there is another Alien species that lives inside gas giants. Due to the extreme gravitation pressure no race could be thought to live there.
The gas aliens attack from other gas planets, whose ships are pretty much impervious to any of the weapons from the humans or the allied Alien race -- only one ship becomes slightly damage when one of the Alien captains sacrifices his ship and crashes into a gas aliens ship.
This gas species sent an emissary vessel to see the human King - which was a transparent pod with swirling gases which was described as to have a vague shape inside, due to them living at high pressure they need a container - to deliver the ultimatum that no further action would be done as long as no one went near the gas planets or tried to harvest the gas again. When the King doesn't accept their threats and implies the emissary isn't going to be allowed to leave the emissary says they didn't plan to, and the pod splits open around the middle and the pressure blasts out killing the King and everyone else in the throne room.
The rest of the story continues with the boy getting inaugurated to replace the Human King, and the human colonies meeting to try decide to handle the this - without the harvested gases there would no longer be interstellar travel. The human harvester people resort to suicidal runs through the upper atmosphere to collect and flee quickly before retaliation. The Alien king resorts to kidnapping the plant planet girl to get her baby, seemingly to get a way to be able to communicate directly to the gas aliens.
Hope thats enough, it was a fairly large book directed to mature audiences.