Pretty sure turtles can spawn in any water that's connected directly (that is, nothing blocking the path) to outdoor water. Your fisherdwarves will scoop them up if they do get in. Vertical Bars probably block anything in the water except the liquid itself.
Turtles will actually randomly appear in nearly any body of water on the map, regardless of it being connected to any outdoor water source. Turtles are vermin, and vermin teleport around randomly to any spot that's an appropriate biome for them. Until your map runs out of turtles, which happens pretty easily.
Can I direct my dwarfs to clean up muddy floors?
Dwarves with the 'cleaning' labor enables will automatically clean smoothed or constructed indoor floors. They won't clean rough floors, soil or sand, or outdoor floors.
Can I make the dwarfs stuff all these bags into a chest or something? Seriously I have tons of empty useless bags from buying sand.
Empty bags go in a Furniture stockpile. They don't get stored in chests or bins unfortunately.
Do fluids trigger pressure plates? If so, what depth does the fluid need to achieve before the plate triggers? Also, I'd assume that normal rules for magma-proof-ness would apply to pressure plates?
When you build a Pressure Plate, you select what triggers it (magma, water, citizens or any creature) and the level of magma or water it triggers for, or the size of the creature. And if you want a pressure plate to survive magma you need to make it using mechanisms made from magma-safe materials.