Xenophiles should probably be rather upset if you freeze or cannibalize inhabited planets (possibly even with pre-sentient pops), but otherwise be positive towards creating ringworlds (ideal living for all kinds of aliens seems like something they'd like) and possibly Dyson spheres (more energy readily available = better living (or a bigger Purification Fleet, but they don't need to know about that)). They could see the Sensor Array as something positive as well (because you want to learn more about the other species' activities, surely to advance peaceful co-existence!), and might be moderately impressed by the Science Nexus (because clearly it won't be used to research weapons!).
Spiritualists would presumably dislike cannibalizing Tomb Worlds as well, though freezing them might not be seen as quite as bad. As mentioned before, if you are foolish enough to mess with the systems where their Holy Worlds are, there should be no forgiveness, and, if you have Leviathans, they probably should Awaken right there and then and set out to eradicate you, regardless of the number of Awakened FEs, Prethoryn/Unbidden/AI troubles, etc. Other than that, Dyson spheres and ringworlds might not attract their notice (unless they are of the opinion that messing with the galaxy's nature as a whole is bad, in which case terraforming probably should be offensive too), the Sensor Array would probably not impress them very much, and the Science Nexus should probably not get much notice.
Materialists could either react in a friendly, if condescending, fashion ("It is good to see that you understand the importance of science, even if your idea of science is laughable! Come see us in another thousand years, if you are still around by then, and we might consider you somewhat close to an equal!") or be rather upset ("Your species is already researching things best left alone, and now you seek to expand on that? Your 'Science Nexus' will be allowed to stand, for now, but if we see even a hint of AI research or experimental FTL drives our mighty fleets will pay you a visit!") if you build a Science Nexus. They should probably be mostly positive/wistful when it comes to ringworlds and Dyson spheres, though (unless you have gone out of your way to insult them), and ignore the Sensor Array.
Xenophobes should probably be dismissive or negative towards everything you do. The Sensor Array, as mentioned above, might be viewed as a threat as you seek to gather intel on them, as might Dyson spheres (because more energy = more ships = more likely to get uppity) and the Science Nexus (because better tech = better ships = more likely to get uppity). Ringworlds, if sufficiently distant, might be ignored, but if they are constructed anywhere close to the FE they should likely be viewed with suspicion (because even if you don't colonize them right away the FE wants to be left alone).
