$100 USD is to low to start trading. That amount is just good enough to hodl one or two coins for long term. You made the right decision to hodl ETH because the price is going low and it can recover anytime. ETH price should be back around mid December this year or probably at the start of 2019. If you want to be successful on trading and earn on short term like most traders do, $500-$1000 will be enough to buy different coins with high appreciation rate.
I think you can trade with any sum. But your profit will be commensurable. And for learning it's not bad. Trading allow to work for any people. And slowly increasing own trading funds is a recipe for success and a sign of the right strategy