2) can I have a short position on poloniex during the harfork
thanks
1) keeping coins on exchanges means you may lose coins on one chain. before (the topic by theymos) was about not knowing how the split is going to happen. miners haven't started to signal yet and if 2 chain existed we didn't know which one would succeed and your exchange might have kept the wrong chain.
now we know SegWit has nearly 100% support and there will be one chain only.
as for BCC, keeping on exchange can mean them not crediting you your BCC tokens because it may not even be worth the effort or the risk.
2) shorting is a different story. are you expecting price to fall!! if so do it. and live with the consequences of losing a large sum of money because of it
https://www.google.com/search?q=guy+loses+million+trying+to+short+bitcoin