Can't stack two of the same type.
And yes, the target compensation just reduces the penalty - so it won't help if the target is sitting still. The penalty can get pretty big though. It's -1 every 20 DPR, so a mecha moving at 200 will have a -10 penalty to hit. Even with the class 4 compensator that's still a hefty -6.
Heres something I've noticed in a recent game - the penalty for more skills than you're normally allowed is only for increasing them with XP - training via money or skillXP from usage uses the unmodified amounts. So if you wanted to, you could increase your basic set of skills with XP (focusing on things that there isn't a trainer available for), then run and train all the other skills you want with money in order to get as many as you want. So, for instance, I could start with basic mecha combat skills and some of the hard to train ones (like intimidate, only police train that one - or code breaking, don't think anyone trains that one.) and get those up to 12 or so, then run out and train all the other skills - spot weakness, insight, repair, medicine, first aid, ranged combat, close combat, dodge, electronic warfare, etc - up with money. My cost for raising anything with XP would be huge, but as long as I had money (which, honestly, isn't that hard to get once you know how) I could get anything trainable up and have as many skills as I wanted without needing high knowledge or the polymath talent.
You could use most of your XP just improving stats. If you really wanted to go nuts you could raise one stat 20 points for 105k xp, or two stats 10 points each for 55k xp. Plus the costs of raising those early skills that you can't train, and you're not looking at a whole lot of XP left over to be training all your skills with anyway. And the notion of just abandoning raising skills with XP after a certain point starts looking more and more attractive. Especially if you eventually get a really badass mecha and stop grinding xp after about 100k anyway, thats the point where you'll have all your stat gains and talents and such.
I think my next char will be a "master of all trades" using loads of money for skill training
I'll just reject the whole notion of max number of skills and learn everything
Btw, for anyone wondering about heavy actuators: Their effect on how much you can carry works like this:
(Total points of heavy actuators) / (Size of mecha). Max of 2. Divided by 2 to get the tons (each point is 1/2 ton).
So a size 4 mecha with class 8 heavy actuators gets the 1t bonus to carry weight. Size 8 would need 16 heavy actuators.
The size 4 would carry a max of 7.5(base) + 2(size/2) + 1 (actuators) = 10.5 per MV/TR penalty.
The size 8 would carry a max of 7.5 + 4 + 1 = 12.5 per MV/TR penalty.
Considering the mass of most of the heavy actuators, this gets expensive tonnage wise as the size of the mecha goes up. Assuming you buy the store bought + variants, the size 4 would need two +3s and a +2 at 0.5t each, so 1.5t of actuators to carry an extra 1t of carry weight per MV/TR penalty. Considering the overall weight of the mecha, that would let you get 0.5t extra if you hold 20.5t of carry weight, or 1.5t extra if you went with the -1 penalty and held 31t of carry weight.
The size 8 needs 16, which would be two class 5s at 1t each and two class 3s at .5t each. So 3t of actuators to carry an extra 1t of carry weight per MV/TR penalty. In this case if you didn't hold enough for a penalty (sticking to 24.5t carry weight) you would actually lose 1t of total potential weight. Going to -1 would break even on extra carried weight vs weight of the actuators. You would need to go beyond -1 penalty for carried equipment to make it worthwhile to install the actuators instead of just installing the equipment.
Plus in either case the extra mass on the mecha is going to be slowing you down.
Ofcourse the actuators also do other stuff - for every 10 points of them you get +1 throwing range, for every 4 points of them it makes melee weps do +1DC, and they also boost your mecha's walking speed (which can be useful if you're not in space). But if you're focused on mecha gunnery style weapons and your mecha doesn't walk, you only really want them for the extra mass they allow you to carry.