If I recall, scaling is equal or better from level 6-8 (depending on type of rune) onwards for scaling runes compared to the flat version, and while getting to level 18 might be rare in pro play (and maybe rare-ish on supports in regular play...although I find many solo queue games do drag on to level 18 full build slugfest since people are bad at actually ending), it'd be extremely rare for someone to not get to level 6.
The trade off is you're weaker pre-6 of course, which at some levels of play is enough of a difference for the enemy to get an early lead and snowball off of. If you do believe you'll get through to 6 without issue, then scaling is probably the better choice (although consider you might be better able to start snowballing sooner if you had the flat version).
Another thing to consider is how much you actually need the raw stats early on. You almost never see scaling AD runes on adcs for example, since people want to be able to last hit effectively early on
I don't feel mixing flat/scaling of the same type is worthwhile. I'd rather plan for the snowball or plan for late and actually be good there, rather than be just ok. (Although tbh the impact of runes onm the whole is probably pretty small compared to raw skill differences.)
Quints are generally viewed as most useful as flat bonuses, just since the size of the raw stats given by a quint makes a relatively big impact on early game more so than the smaller numbers involved with the other rune types.
edit: For those with lots of rune pages for micro-optimization, scaling versus flat defenses can eb a choice made based on your lane opponent(s). If you're against an AD opponent, you're probably better off with scaling mr instead of flat mr for example since by the time teamfights (involving an AP opponent) start, you'll be past the cross over point, and similar for versus an AP with armour runes.