It won't melt the coke bottle but it will seize a power head, if in doubt just add 100gms at a time, dissolve and let it cool or I just dump it all in without mixing it and let it sit for 15 mins and give it a stir up at this point.
The B works out a lot cheaper than the bottles and you do not need to mix up the whole lot in one go, there is a 20gm scoop in there, just 5 of those per liter.
Having said that and having used the red sea stuff for 5 years and changing over to the stuff 100% reef safe sells, there is no difference between their b mix and the bicarb 100% sells.
The A mix has strontium in it and there does seem to be some benefit in that, the c mix is also a mix of Mg chloride and sulfate so it seems to keep your ionic balance better than using epson salts, but you can follow randys revised mix to fix this.