Floodgates do not crush anyone, and do not get deconstructed if they are blocked. If anything, including a butterfly corpse, blocks a floodgate, it simply doesn't close when triggered until it is cleared. Once cleared, it will close (unless the lever has been triggered more than once, in which case you may have to flip it a few times to fix the gate). For this reason alone, I consider floodgates unsafe for blocking flows. Why? If an aquatic or magmatic creature comes running toward your fort or if crap such as bones or chunks are caught in the liquid, it could jam the floodgate open.
A 1x2 drawbridge (meaning it raises in a direction rather than retracts) acts exactly like a floodgate when raised, forming a tight, liquid-restricting barrier. When lowered it acts as a floor. Beware of accidentally crushing things, I like to put a wall grate or vertical bars to restrict access to the gate.
A drawbridge made of magma-safe materials is magma-safe, as long as the mechanism which is attaching it to its lever is also magma-safe. What do I mean? Press "t" and look at the bridge. If the mechanism listed as "inside" it is magma-safe, you're good. The lever and the mechanism "inside" the lever DO NOT have to be magma-safe as long as they never come into direct contact with magma.
Magma-safe mechanisms must be made from Bauxite or Raw Adamantine, nothing else will work
I like to mod in a smelter reaction to cast iron and steel mechanisms. It looks like this:
[REACTION:IRON_MECHANISMS]
[NAME:cast iron mechanisms]
[SMELTER]
[REAGENT:3:BAR:NO_SUBTYPE:METAL:IRON]
[PRODUCT:100:1:TRAPPARTS:NO_SUBTYPE:METAL:IRON]
[FUEL]
[REACTION:STEEL_MECHANISMS]
[NAME:cast steel mechanisms]
[SMELTER]
[REAGENT:3:BAR:NO_SUBTYPE:METAL:STEEL]
[PRODUCT:100:1:TRAPPARTS:NO_SUBTYPE:METAL:STEEL]
[FUEL]
As for making the floodgate\bridge, you can use Iron, Steel, Nickel, Bauxite, and a couple other materials. I usually choose Nickel because it's magma-proof and otherwise useless. Save your bauxite for mechanisms
Someone earlier mentioned that, attached to the same switch, a drawbridge is closed when a floodgate is open. This is very useful for computing, failsafes, traps, and other creative contraptions.