Yes they would get along... usually its the same generas that don't... but your 70 gallon tank will proove way to small for these two fish. A tank twice that size would be sufficent. Remember yellow tangs get 8 inches (and amost that wide) and blues get 10-12! They need 6 feet of swimming room minimum... I have my two tangs in my 125 gallon and they will eventually need a larger tank IMO
... as for getting small fish and letting them grow, you'd be able to keep them in that 70 gallon for 2-3 years before giving them away/selling them.