For lvl 6 I wouldn't be too obsessed with the Nova reactor, since there are other good options. As the Model shield is the prime contender at 6 you might wanna check out stuff like the Warriors Pride reactor. Higher cap, slightly less regen, but a shield regen buff and the very nice shunt to reactor effect (although this was broken in ST3).
Sure, it's only 10 levels from Reactor 6 to Shield 7 and it might be an idea to decide on which way to go before you get there (i.e. if you are gonna go with Barrier at 7, then just stick with Nova all the way)