What are the main benchmarks for a fast science victory?
There will be some debate but in essence science victories are best with 15-20 cities or more which realistically means violence. My general violence benchmarks are
10 culture per turn by T25 (a target, not a reality) but basically a government by T50, earlier is better. This first step ensure earlier feudalism which is primarily for builders but also knight.
7 cities by T70... the is easily reachable, 10 by T100 was the old benchmark but in reality for fast victories apart from peaceful CV it is too few cities. 7 by 70 is roughly what I always have but you should also be taking out lots of cities by this stage so you are at 15-20 around T100.
At around 80-90 you will be pumping out 6-7 charge workers for chopping in libraries/universities so yours science heavily escalates, you envoys at this stage will be concentrating on science and culture CS. You should also be planning for Kilwa Kiwisani, Rocket sites, eurekas and inspirations further up, and other wonders you may want like Big Ben as you need good cash, if you can get a lot of cash you may be able to shave some turns off your victory.
You should already have pyramids/coliseum amd have good culture growth for the moon landing push.
You should also be looting, placing specialists and managing city size and use.
You say Ruhr, I tend to go Oxford university as if you have done most of the easy eurekas this can give some nice surprise which will shave off a few turns. No Idea why people get Ruhr as rockets do not need any city production really. Buy the platforms, buy the builders and have enough chop for the first, second and last rockets.
It is all about shaving turns off, doing A shaves 2 turns off, B shaves 3 turns off. There are lots of turn shavers, every eureka and inspiration does, chopping at the right time, looting to finish inspirations and eurekas as long as you understand what overflow you loose.
How many cities should you have by turn 70/80 and what is the best way to go about getting them?
As above, violence. Because 7-10 by T70 will not be enough cities.
Harvest and monumentality help but are not critical, having 2 armies and flat ground helps. Taking cities means you capture things you may not build otherwise... so capturing 2 harbours gives a good eureka, helps with an inspiration or 2 and is a turn shaver. Making the city and building the districts is so much more work and time that capturing cities is a big time shaver.