I think it should be kept as it is... Just to aviod confusion. Plus, the additional few words doesn't take a long time to read through. For example:
If not changed to 'immunity to Poison-type attacks', it could be misread as 'immunity to poison' (the status). Some readers may not be able to tell the difference between Poison and poison.
Then again, about the bulky Waters thing. I support the 'bulky Water-types' convention though. The word 'Waters' does not signify the group of Water-types. Lets say, 'Starmie and Staryu are Waters'. This sounds weird as compared to: Starmie and Staryu and Water-types. Probably the reason why it is 'bulky Water-types' and not 'bulky Waters'.
That's just my stand though. I won't trade 'fluency' for 'correctness' unless both are correct. :x so, what is the correct/better one?