Ok, Ok, Ok. I hope that I can remember everything you are asking. With some soil, or soilless mixtures such as ProMix have PH buffers that hold the PH at a certain level, but after a while of watering at different PH levels and considering that different nutes have different PH buffers as well the PH in a particular soil can change, some do, some don't. The safest and easiest way to deal with this IMO is to try not to worry so much about run off PH, unless you have some plant problems that you can not figure out, but instead, PH your water after you add your nutes, that also answers your question about mixing them, yes you can mix PH Up/Down into your water/nute mix. Always PH after you add the nutes. I think that during the clone stage a higher PH of about 6.8 is more appropriate for the plant and as you get further along in veg towards flower then you can get down to about 6.2/6/3. I have done this every since I have been growing, know others that do it and Zandor said basically the same thing on his grow report about soil PH levels. To avoid salt build ups that would give you a reason to need to monitor the PH runoff, just give your plant a good flush about every 3 to 4 weeks if you are really worried, Salt build ups can depend on the nutes, some nutes are prone to salt build ups and need to be flushed every 3 weeks or so, but in my case I flush once in veg, once right before I switch over to12-12 or right when I start up 12-12 and again right before harvest.
Hear is a big mistake that many people make that makes the plant mis a beat and can affect the life cycles , when switching over to flower some start giving bloom nutes as soon as they change the light cycle. This is not what you want to do, because the plant takes 7 to 10 days to actually make that change from veg to flower and when they are in veg and start getting steady doses of bloom nutes it can make a chink in the chain. Also, when I switch over to 12-12 I do a 24 to 36 hour dark period. May be a hippy tail, but I start seeing sex about a week later and they seem to just swell up and take off when I slap them into flower with the HPS. With some strains I am getting nice bud development by 2 weeks.
Another cheep thing that can help during flower is the use of Molasses, I have an article in the organic growing section on it. It can assist in the break down of certain nutrients in your furts in such a way that it is easier and more efficient for the plant to process them.

Well hope all of this helps some. I am sure I forgot something, but that is the story of my life.
Adieu