It’s certainly something that deserves study. You are right the faster you get DPS on your target the more effective DPS you will do.
One drawback to mixing drones is different speeds. The Valkyries will get there much sooner than the Hammerheads.
As far as Drones go, the current best overall drones (speed, hp, gun range, etc) are Amarr, but they are EM damage and not very helpful against Armor Tanks.
In my Pilgrim I carry a set of Infiltrators and a set of Hammerheads so I have a choice, however switching Hammers for Valkyries would probably be a good choice to better deal with Armor Tanks.
As a very general rule Thermal is the best overall damage to do when you don’t know what your enemy has. But it’s much better to chose your damage type to fit the weakness of your opponent.
This is why you see me checking ship info before and during fights so that I can confirm the resists and likely weak points.
Also, since the range you engage at is usually less then 30km from drone drop to target speed isn’t a huge factor. Perhaps 2-5 seconds.
Having thought it through while writing this, I think the best way to go is with EM drones and Explosive (Hammers and Valkeries), so you can chose based on the target.
Mixing while providing a broad catch all would not be optimal.