The exact nature of Time Masters and what they can, cannot, will, and will not do was left pretty fuzzy so far. (The comparisons to Doctor Who here are practically inevitable).
But, based on the comments Rip Hunter made, both about the team he selected, plus the one civilian he chose to interact with, as well as the comments from Chronos, it seems like the primary concern here is that taking someone important out of their timeline and risking their life could have far-reaching implications beyond the changes Rip wants to make.
It seems that these Time Masters have some way of determining how much of an impact a given action might have on the future. We can only assume that Rip Hunter, given his training, has determined exactly what he can safely do to Vandal Savage without risking an even worse outcome. For example, if he were to take Oliver or Barry and one of them were killed, there might never be a future Justice League (as we saw mentioned briefly in newspapers from the future), and that might be worse than what Savage did.
Another important issue, hinted at near the end of the episode, and which we've already seen happen in The Flash, is that time itself has some kind of "damage control" mechanism: when Barry went into the past and changed things too much, it caused ripples of chaos that spread out far from the originating event. Rip presumably is aware of this problem, and even warns his team that, in some cases, they will be fighting against "time itself".
So, ultimately, Rip is just being as extra careful as he can, to minimize the impact he has on the future timeline. He's already
acting against his orders
by even trying to stop Savage in the first place. He doesn't want to do anything to cause any more damage to the timeline than absolutely necessary to achieve that goal.