Hello,
Are there any other exchanges aside from Poloniex that offers more variety of Altcoins for margin trading to short/long etc?
yobit.net also offer variety altcoins for exchanges. But I will advise you to used poloniex as it offers more users friendly platform. You can also google for exchangers that has variety of altcoin margins trade and you will have many verity of option to choose from.
I think yobit.net is also more profound when it comes to exchanges. There platform is also very easy to use and they have more than 100's of alt coins listed which you can exchange easily. There user interface is very friendly (however sometime it takes long time to load) but when it is loaded completely then you can move easily within the exchanger. They also have live chat box where people keep talking about top coins so you can invest there to earn more of it.