Merged mining scrypt coins
a) no need. you can fork your chain with the new chain-id at any time. but the wallets have to know the point of fork. The spanish coin pesetacoin forked the chain  near block 32000. and has no problems.

c) there is (at least) one pool mergemining with 3 main chains (you have to choose one) and 4 aux chains (pesetacoin orgcoin hutercoin and usc) have a look at http://manicminer.in