For example, someone told me it's nonsense to hold btc and ltc, because ltc follows btc pumps/dips and I should only hold 1 of them. Should I consider adding a privacy coin? Or even more "platform" coins like eth?
Nobody can be really so sure in this cryptocurrency arena. One's advice is as good as the other one. You should stick to your own strategy and adjust it from time to time when needed. I would add that you make a separate funds for buying new ICOs which have more chance of doubling or even quadrupling in value and then release them into the market. This has been working for many and this is partly one reason why the ICO platform will be here for a longer time. Anyway, good luck on your endeavor.