Can someone confirm if i am ok to double spend only part of my initial unconfirmed transaction
Yes. The moment you get a confirmation transfering one of the inputs to another address, the original transaction is no longer possible.
How many blocks would it take to confirm if i used a fee of 0.001 for 0.02 BTC (1558 bytes) transaction?
That's 128 satoshi/byte. My Bitcoin Core estimates it'll get confirmed within 24 hours (144 blocks).(see my next post)