Well. Now you know one of the many ways *not* to breach a volcano. Just keep at it till theres a minimum of burning short-arses and yer golden.
Incidentally i'm lazy and use no floodgates, fortifications or grates or any such but suffer no magma critters. I cut the hall for the forges and plan where each workshop will go, channeling the required holes to let the workshops access the magma. I then put a down stair in one corner of the room and cut the radiator (the grid-shape that will contain 7/7 magma) beneath.
I use legendary miners to cut the actual flow loose, i2amroy gave a trick on page 1 that will never cost you a dwarf, i always use that (designate a nearby tile for mining after the dwarf begins on the desired tile, undesignate it again once hes breached the pipe and is moving, if you have multiple miners you may need multiple of these 'bait' designations to be sure the guy tapping the pipe gets assigned one of them)
Another reason to use a miner for this is that if through some stupid dwarfyness it all goes pearshaped, he can cut himself a new upstair faster than magma flows and you just have an extra stair to deal with, as opposed to a burning beardy.
Once the magma is running into the radiator and the miner is out, i channel out the downstair from the forge-deck and floor it over. When you then build your workshops they will block the holes you cut to provide them with power and there is no way for critters to get onto the forge level, even if they drift into the radiator (which they will almost never do since theres no path). Just make sure you are positioning the workshops so that impassible tiles are above the holes allowing access to the magma, if you mess this bit up then yeah...crabs and gimps everywhere.
The only true drawback to this kind of method of inflow is that you cannot drain the radiator to expand it, though you can cut another one adjacent and then breach into your original one using the same tactics as tapping the original magmapipe. The floodgate is usually there for control for just these reasons, but magma-dwelling building destroyers then necessitate all that faffing about with fortifications and grates, which is just timeconsuming. Without those features the entire magmaforge including it's 'plumbing' can be dug by 1 miner, probably within a month.
If you decide later to build a superdwarfy magma weapon then dont make the mistake of linking the reservoir to your radiators, build it it's own tank, magma flows much slower than water and charging your weapon will likely depower the forgedeck