You can try your luck on Yobit with as low as 10k sats but you can't deposit less than 0.002 on the site. If you can afford to lose that amount you should try it for getting some experience at least, but first you should say good bye to your money and if you feel that you are okay with that than try to trade, why not.
I think the minimum should be the amount you can lose without stress and you need it. I never try trading myself with the Bitcoin price variation, but I'm waiting to a good amount before to give a try.
I would say the minimum should be with 0.01 Bitcoin from my point of view.
Depends on how much you want to earn. If you don't want to risk much, you sell your altcoins when the price is 20% up, or even when it is just 5% up. Waiting more is very risky because you might miss the time when the massive dumping starts.
Now, if your balance is 0.01 the maximum profit you can have without big risk would be 0.002. And I'd say you are very lucky if you'd managge to get even that profit.