Before the accelerating universe makes it impossible without FTL to reach other galaxies the advanced civilization in my story aims to create their own super system, which will remain gravitationally bound if expansion remains constant, to the end star formation era and the second stage will survive till the end of the blackhole epoch.
Two systems will be created at a distance close enough that expansion will not separate them in a short time (on a cosmological time scale) but not close enough that they will gravitationally disturb each others stability.
System 1 is a massive galaxy containing 500,000 trillion stars, which for reference is 5,000 times the size of the largest known galaxy IC 1101, the number has been taken from an estimate of there being 250,000 trillion stars within 1 billion light years, the empire has a radius 3 billion light years, red dwarfs will be the majority of transported stars as they can live for trillions of years, it is estimated that 75% to 85% of stars could be red dwarfs.
The number of 500,000 trillion is a rough rounding down of the estimate of suitable stars that can be brought in, this number can be changed if limitations require it.
I have read that there are no limits to the size of a galaxy as long as enough dark matter is present and its density increases further from the core to stabilize the orbit of outer region stars. Following on from my previous question, the method by which the necessary amount of dark matter is transported is undecided but we can assume it is possible.
System 2 which will eventually join system 1 is a system of merging ultra massive blackholes, this is not relevant to the question but it means that that system 1 does not have a central supermassive blackhole, any blackholes that form from new stars created in the transported interstellar medium will be merged with the BH system. The lack of a central blackhole should provide more stability at the core so less turbulent orbital paths should occur.
I am wondering if there are alternatives to the single galaxy of system 1, I had thought that arranging galaxies in a stable orbit so they appeared as a super galaxy of orbiting galaxies but that may involve an even larger dark matter disk and halo which surrounds all the galaxies, also they would likely just merge creating turbulent orbits meaning most of the matter could be kicked out again. An idea like this would be more interesting and visually pleasing to me but seems much less stable and involves too much dark matter.
After the last stars have burned all of their fuel they will be fed to the remaining sole ultra massive blackhole and the civilization will live the rest of their years with very low energy consumption provided by the blackholes radiation but until that time they wish use as much energy as possible provided by stars in the one location together.
Are there any arrangements for this number of stars and volume of dark matter and interstellar medium that would remain stable and bound, other than as a single massive galaxy? Other than collecting and transporting the matter what other issues will the single galaxy or other options face which could put limits on their size and stability?
Additional notes for other arrangements - Something I did not address in the question is the high rate of kick outs in galaxy interactions, this is why it could be messy to cram a large amount of galaxies in a small area, even if they have stellar engines to correct positions this will still be a big problem without a stable arrangement, expansion is not an issue for gravitationally bound structures, which the system will be, this high number of kick outs is what occurs in rich clusters which have a large number of galaxies in a relatively small volume of space.