I don't know of any problem with a hydrogen helium mixture. Perhaps mostly helium at liftoff; then use hydrogen as the top off gas. Both are bad about leakage. At present, supply exceeds demand, so helium is cheap. Huge balloons would quickly make helium in short supply. It could cost 100 times the price of hydrogen. Hydrogen production can be scalled up, but doubling the helium production, would involve extracting it, where it is only a few parts per million, and in small quantities, where the other gas in the mixture is also costly to purify. Dumping unneeded gas of most kinds in the atmosphere will soon incur the wrath of environmentalists.  Neil