Ya, the reactor space thing is an industrial craft issue I think, because it uses a function to determine the size of the reactor [3x6 + 1x6 / reactor chamber]. Or else red power would already work perfectly with it, as tubes avoid full inventories as well. If you're using buildcraft for it, you could just use an obsidian pipe where they land and stuff it back into the filling pipe (using an iron pipe obviously so it'd go the right way.)
I found that a bit annoying as well. I tried to find a way around it, and even looked at the code, but I don't think there is since to use buildcraft and/or redpower to add items to the reactor it can't be 6 chambers, so the reactor could potentially always have more space. If we could pipe into the chambers instead of just the core, then it'd probably work perfectly and we wouldn't have the buckets flying out of the reactor.
The buckets have to keep coming the slightest delay (in this case the time it takes from chest to the reactor) will meltdown the reactor. You cannot wait 10 sec for the buckets to arrive they need to be spammed all the time to prevent delays.
Also lower power generators arent exactly safer. If it bugs then it will explode anyway even with 1 uranium. So to keep it simple:
Cooling system working: No meltdown
Cooling system bugged: Meltdown
Less uranium would only increase the time it takes to meltdown but since you arent babysitting it all the time this is quite neglictable.