does the planet quality number mean anything other than simply the number of tiles you have to build planetary improvements on? Does it mean anything else?
No, but keep in mind that planets with PQ 11 or higher get a 10% approval bonus, and planets with PQ less than 8 have a population growth cap - you can ferry people onto the planet, but the population won't grow past a certain point on its own.
so far, the only "victory" conditions I've disabled are "technology" victory and "ascension" victory. I had previously played with "technology" victory turned on, but I thought it was taking away from the quality of my games. Do you feel that disabling "technology" victorys is a good idea or no?
I play with it off, but it's pretty much a personal choice. Especially on big maps, it's quite easy to finish the tech tree entirely before taking over the galaxy.
In my current game, I'm ahead in the "advanced/extreme colonization" tech race because I had a high quality, barren world in a solor system within my influence borders that I wanted to colonize. Now I notice that there is a purple star planet (PQ23) that requires advanced colonization to colonize, but it is deep within Acrean influence; in fact, it's in the same solar system as two Acrean planets (neither being their homeworld). I could probably colonize it before they do because I'm ahead in extreme colonization; but my concern is that this would be just a waste of resources and tceh investment because the planet would culturally flip to the Acreans. Should I pursue colonizing this high quailty barren world? also: does the fact that I have excellent relations with The Acreans (we're allied) have any effect on this situation?
That depends on the surrounding planets, as well as exactly how close this planet is to your culture, how many planets the Arceans have nearby, and several other factors. If there are a few lower PQ planets nearby you could take as well, you probably won't have a flipping problem. Sticking a few embassies on there temporarily wouldn't hurt either, especially if you can get it to full population pretty quickly. If nothing else, build an influence starbase touching the planet, and upgrade it as far as you can with + influence modules - unless the Arceans are very densely settled in that area, that should protect you.
Relations with the other civ don't effect flipping at all as far as I can tell. Planets flip between allies and those at war, and pretty much at the same rate. Settling planets in someone else's territory (or allowing them to settle in yours) can actually lower your relations with them, as you get a negative relations factor for "close borders". This is put in the game to prevent us from abusing friendly AIs by taking them over culturally. However, since you are allied with them, they won't object when you start settling their space or building influence bases to defend your planets.