Please stop sending BTC to your old BTC deposit addresses. The funds will not be lost, but all new addresses must be generated, as I must assume the old wallet is compromised. It is very unlikely that it was, but "unlikely" is not good enough.
Does this accident affect other, not-BTC deposit addresses? My rigs send regulary small amounts of coins directly to my poloniex deposits, do I need to stop sending now?
I guess that depends on how safe you feel with unlikely. Going by what he said, you should be good but it's best play it safe and stop using all old poloniex coin address as I would assume the wallets were all on the same wallet server (could be wrong).