With regards to your question about buying altcoins, it's up to you :) With BTC being at an all time high price, the best advice is to just sell SOME off. The amount you want to sell is up to you. If you want to sell cash or buy up altcoins is also up to you. What I did, was sell 10% of my current btc holdings in cash, and started buying up altcoins with about 3-5% trades.
Now you mention you have a big position in Steem. I'm not sure how much, but try to limit position to no more than 20% of btc account (that's even big). It prevents events like what you're experiencing from happening where being stuck on a trade because a bounce failed will prevent one from taking more trades. Of course, this is dependent on account size; smaller account sizes, tend to hold bigger position so you can grow your account faster, and bigger account sizes, start using smaller positions relative to size for diversification.
In your STEEM trade situation, the best approach when the coin keeps dropping is to buy more and sell on the next bounce. A buy of 10% below your initial buy at 400s would put your position at an average of 425 but your position has now doubled. If you sell everything at the next bounce close to 450s which is your first buying point, you would get a TOTAL profit of 5-6% which would be the same as 10%-12% profit on 1 unit position. Now I know I'm seeing the results on chart and easy to say, but that is the strategy I take regardless if I have a position and the coin keeps dropping. Hope this helps.