Is there any danger / risk having my bitcoins at poloniex after 1. August ? Also will my current Bitcoins remain Bitcoins (and not BCCs or a mix) at poloniex and bittrex after 1. August ? Cant find the answer anywhere !

Let that Aug 1 thing aside, you are always at risk if you hold your coins on the exchanges. Look what happened to btc-e... poloniex Also randomly seizes their users money, locks their accounts. I have No information about bittrex though.
I think not all trading sites are at risk, but if for poloniex used to be very safe for trading and withdraw, there is no constraint at all but for now I see many people complaining about this poloniex site in terms of withdrawal.
Maybe I think Bittrex trading is very good and safe to play trading and do withdrawal, because I from the beginning until now still use Bittrex to play trading and there is absolutely no problem at all for that.