I'm probably gonna say only you can do your own bits because otherwise we're gonna have the dedicated limb man and then a bunch of super troopers
The main idea behind multiple types of limbs and hands and such is versatility more than just hitting loads of times. Having a sword, a gun, a club and a poison stinger, for instance. So extra hands wouldn't be needed as far as I'm concerned. We could change it to be you can only attack with one particular part or weapon per turn, but in the case of say, an enemy that is weak to a certain weapon or damage type, it seems silly to say "Yes, I know you can hit fast enough to strike 3 times in a row, but no, you may only use your demon bane sword once. The others must be kicks."
The idea behind the speed is really that in order to get off any significant number of attacks, you really have to be fast. An 8 armed monster will more have the advantage of lots of kinds of damage and being just a big damage sponge. Like DF cave spiders before aimed shots where you just cleaved at a million legs for eternity while they tore you apart. And really, part of the reason I like the gamble method better is because it kind of makes sense to me that if you attempt to do a bunch of attacks you might end up fucking up. Over extending yourself. Maybe less have a back fire and more the enemy just gets a counter hit? But the idea is that if you fail a roll then your sequence ends. You can't just line up 10 attacks and then fail 7 and determinatly chop yourself in half with a half dozen self attacks.
As per the giant parts, What I would probably do is like, the individual parts of the giant arm would be a single slot. Like a giant fist is one slot. But like, two parts? Maybe 3 slots. And a whole arm? Maybe 4. Because a giant arm that reaches below your knee isn't really normal right? But not THAT abnormal, so only a little outside.