Has anyone experimented with a "mixed" All-X strategy?
If by "mixed" you mean starting with one strategy and switching to another mid-game, then yes, it's been done. If you just mean building all of one building type on specific planets... that's not exactly an all-x strategy.
The all factories or all labs (or all econ) strategies get their strength from setting your sliders to x/x/0 or 0/0/100 and gaining the full production/research bonus from your buildings. If you do it on a planet by planet basis, then you've probably got your sliders more evened out, which means you are not going to get anywhere near the research/production you would in an all-x strategy.
This is not to say that focusing planets on one kind of production in a normal game is not beneficial. If you're not using an all-x strategy, then building one specific building type on a planet (like making your industrial capitol an all factory world) is still quite useful.