Heya to answer your question, as long as you are within your 90 day timeframe, you can level up as many characters as you want. they will all have the triple exp bonus. (lots of people here level up multiple teams)

and as for tanking with the druid, it's doable as long as you setup your macros correctly, which will take time to do. But really you can do just about anything with any characters as long as you can setup their macros properly. but in terms of for leveling, you really dont neeed a tank unless you're going to level through instances. with all quest mobs, you should be able to 1-2 shot them with your team, so a tank is not necessary.

good luck on your team