I'm having trouble keeping people alive when they keep rushing ahead

Let the DPS die, ask them to stop, or vote kick. Only the tank should pull, regardless of the difficulty (like all rules, there are exceptions but it should be the tank who is determining those). You should treat the heroics as training for M and M+. Throw Bestow Faith on the Tank right before it charges in. You will then have the option to pop Beacon of Virtue before it goes off if the DPS took splash damage.

is it okay to keep tank around 70% health while focusing most of heals on dps?

This is perfectly fine and absolutely expected ESPECIALLY with paladin and DH tanks (and to a lesser extent, DK). The only tanks which really benefit from topping off are wars and druids and many druids don't need it, either. Your responsibility is to learn the portions of the fights which the tank's self healing won't be able to get through. DPS, however, will take splash damage. Some of it is unavoidable and you will want to make sure that they are topped off as soon as possible. If a dps is taking a lot of damage from avoidable stuff, just send them a tell. 90% of the time people do want to get better and will listen to you. The other 10% die or get vote kicked. :)