for pressurized water to flood to higher levels it first need to make all water on given level 7/7 right?
right.
even if i put some waterfalls on the given level, sooner or later the pressurized water will make the waterfall become 7/7(ocasionaly 6/7 but mostly 7/7) and then start to flood, however if i drain the water with fortification at the map edge then the water will never be able to get past 3/7 on the fortification tile(it never did last time i did this), so the floor that have the fortification as a drain point will never flood, however if i make a floor with water and the only way for the water to leave that floor is by a hole, it might get to 7/7 and start to flood right?
I am not entirely certain but I believe pressurized water searches for a path to the closest not filled tile that can be reached through directly connected 7/7 tiles and teleports there. So basically yes, depending on the design of your drain you can flood your fort. I am not sure if the 3/7 water filled fortification tile transmits water pressure, if not, everything downstream should be safe.
A reliable trick to remove pressure is a diagonal water channel:
pressurized source
W7WW(wall water wall)
W7WW
WW7W
WW6W
WW5W
unpressurized drain
btw, grates stops alligators but won't stop fishes from comming in right? (my map have carps too... so im not realy sure how to make fishing safe there...)
Theoretically, grates, fortifications and so on stop both alligators and carp. Carp are big fishes that can not be fished but must be killed and then butchered. However I have seen dwarves getting pushed through wall grates by water flow so I guess the same applies to carp and other things:
Once a forgotten beast apparently got sucked by water flow through a fortification into the pump stack of my indoor waterfall. In the end(well, after its end) the thing rotted the skin and eyes off of three of my elite squad and several cleaning dwarves and pets.
Small fishes such as turtles can pass through anything as far as I know, but are never found outside of their biome.
Maybe that is why I never did catch any small, fishable fish in an artificial pond or channel, but then again I never tried very hard.
So, good luck, and never hesitate to flood your fortress for the safety of your fishing dwarves.