Now there is considerable fee for sending BTC.
Poloniex is providing me a BTC deposit address.
Please help me in the following doubts.
(1) I have a few BTC investment/mining sites.
Can I link my Poloniex BTC deposit address to that sites so that my withdrawals from that sites are directly deposited to my Poloniex account?
Nope. You don't link accounts. You need to perform a standard bitcoin transaction and pay the fee, i.e. withdraw from CoinBase directly to Poloniex deposit address.
(2) What is the minimum amount for BTC deposit in Poloniex?
That is, can I send 0.0009 BTC, 0.001 BTC etc to my Poloniex BTC deposit address to get credited in my Poloniex account?
There's no minimum for bitcoin deposits (there are for some other altcoins, ie ethereum have min 1 ETH). But don't bother with anything in range of 0.001, the tx fee could be higher than that and it's a waste of time, I don't think you can place any order for such a low value, and you can't use lending option etc.
If you indeed have few BTC, just deposit more. If you want to test the site, make some trades, provide a loan, you'd need at least 0.01.