Is it possible to safely use high leverage?

Suppose you have an Expert Advisor that works profitably (you had a profitable trading strategy which was successfully coded into a program). Now, suppose you trade 10 currency pairs with your EA (let us assume they are uncorrelated, of course many of them are correlated. Or say you are trading 10 different financial instruments, which could include metals and commodities). Your trading strategy works on a small timeframe, say 1 hour or 30 minute, so your stoploss is pretty small, around 20-30pips. If we apply 3:1 leverage per trade, then our risk per trade would be 0.6-0.9% per trade right? now if you have 10 currency pairs with 10 trades open thats potentially 30:1 leverage. would this be an acceptable amount of risk, assuming the currency pairs are uncorrelated? because we are risking less than 1% per trade, the only difference is that we have 10 times more trading opportunity. Or is this too risky?

Unless the pairs are completely or almost so, you are taking 10 almost identical trades. So your risk is 6 to 9% of your account. It is very difficult to find 10 uncorrelated instruments to trade unless you get into exotics or obscure things like sugar and coffee futures. Remember gold and oil are very correlated to many currencies, but not all the time.

I agree with you. Also, unless you have a good working strategy, it will be a great risk to use about 10% of your account. 10 identical trade would also reduce the risk as well. Only an expert trader can be sure of this.

How many “uncorrelated” currency pairs or financial instruments are there? Is correlation higher in smaller or larger timeframes?

Most new expert advisors these days trade baskets which is very much what you are suggesting. Whether they manage to be profitable or not depends very much on the strategy they use. I have not really seen any stand out ones.