You could stick with the 6500k thruout the grow but your yield will be down anything upto 50-60% I think, T5 tubes ar elike £5 so I would defiently get some come flowering, If its a 8 tube maybe leave 1 or 2 blue tubes in and have 6-7 red, a little blue light in flowering is good to help with stretch but the majority should be red.