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I have a K-type star (0.5 stellar masses) as the central body, and the planets are as follows (those I want to be in the habitable zone in bold):

I: An uninhabitable rocky inner planet 50% the mass of Mercury. Way too hot for life but not enough for automated equipment and bases. Tidally locked.

II: Then further away just on the inner edge of the habitable zone, a hot tidally locked planet that redistributes some of its heat through its atmosphere and has a huge pangea like desert continent facing the star. Life similar to that on a primitive Earth can persist here but flowers most around the terminator line. About 75% Earth mass.

III: A planet about 25% more massive than Mars. Not tidally locked to the star, as with other planets beyond this point. Has an exotic atmosphere with primitive intelligent adapted to the environment. A generally temperate climate with many more island land masses than on Earth.

IV: Almost the twin of Earth, with 90% of its mass. Intelligent space faring life.

V: A super-Earth around 30% more massive than Earth. Shallow oceans with long thin continents snaking across the planet. Intelligent and aggressive space faring life.

VI: Another Earth like planet very similar to planet IV. The species on this planet consider planet IV to be their twin world, and they are often in alliances with IV against the super-Earth in the middle orbit. On the edge of the habitable zone, so cooler than the proceeding planets but kept warmer by its atmosphere.

VII: A gas giant around the mass of Jupiter. Beyond the conventional habitable zone and out of the way of disturbing the other planets but not so far out that effects like atmosphere and tidal heating can't give its moons enough energy to have liquid water. It has two habitable moons each a little larger than Mars among with much smaller rocky moons. These two moons tidally pull on each other a little creating liveable conditions for primitive life, but probably not stable in the long run.

VIIb: The gas giant has an asteroid field co-orbiting in one of the lagrange points and in the other an ice ball around 75% of Earth with a very small rocky moon. Planet is typically around as cold as Antarctica across the planet, with a greenhouse effect from a relatively thick atmosphere keeping it from being any colder.

... So that's the setup I'm looking at. I have the important information about the masses of the planets, but is it plausible to have this system be stable for a long time, allowing for the distance of the gas giant to accommodate not disturbing the other planets that would be in some strict orbital resonance with each other?

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    $\begingroup$ Have you tried throwing this into a gravity simulator? There's at least one OK free one about. Spoiler alert: the more interesting your star system is, the more likely it is to fall apart. And it probably won't fall apart in an exciting, apocalyptic way, either :-( $\endgroup$
    – Starfish Prime
    Nov 12, 2019 at 22:01
  • $\begingroup$ Not sure I know enough about orbital mechanics, but I could give it a go. $\endgroup$
    – Axion
    Nov 12, 2019 at 22:05
  • $\begingroup$ It has a slightly hostile user interface, so getting the damn thing doing what you want can be awkward. It should, however, do an excellent job of answering your question and allowing you to experiment and tweak things yourself without waiting for partially-baked advice. $\endgroup$
    – Starfish Prime
    Nov 12, 2019 at 22:07
  • $\begingroup$ Cool, thanks. Downloaded it. $\endgroup$
    – Axion
    Nov 12, 2019 at 22:22
  • $\begingroup$ Boy, you sure are trying to fit a lot of planets into a very narrow habitable zone. $\endgroup$
    – Arkenstein XII
    Nov 13, 2019 at 0:05

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I finally used Gravity Simulator, and I found out that the Jupiter mass planet couldn't be placed close enough, so I made some alterations. I omitted the Mercury (which doesn't change much I think), and I altered the gas giant to be a 75 Earth mass Saturnian planet and placed it at 1.75AU, beyond the HZ for this system, but I guess it's possible the two moons I gave it could still be habitable through tidal heating and thick atmospheres:

enter image description here

It looks like the system is stable too with these changes. I think you might be able to have three relatively large moons like this.

I then tried to add the co-orbital/trojan planet, and it turns out to also be stable, except it must interfere with the hill sphere of the gas giant, because it causes the gas giant's moons to be ejected. So you can have co-orbitals or you can have moons it looks like.

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