I agree with the "segregated witness" part. Although the "block size" will be within the 1MB limit, is it OK to forget the signature data completely? If not, and someone has to save them as archival data, then I rather prefer them to reside in the 1MB blocks.
Creating new coins however would be a hardfork, no full node would accept it.
SFCs are like bitcoin-2, bitcoin-3, ... and so on. Old full nodes can (and would) completely ignore them. But SFC-updated clients will be able to "see" a new currency type such as bitcoin-2.
You don't even need a softfork to issue alternative currency on bitcoin blockchain. See
The real question is, however, why people would accept your alternative currency?