No, a jerrycan should not leak. They're well built and should stand up to most normal use without damage. The cap seal is removable and easily replaceable. If you fill them with petrol there is an air space at the top that's meant to be there so don't try to tip them back to get a bit more fuel in.

My 2L bottle is something ex military I bought from a surplus store years ago. It's plastic but designed for fuel although you have to be careful filling it. A fuel nozzle will (just) go into it so you have to be careful as you get beyond 1.5L to watch out for splashes. Sorry, can't find a pic of it at the moment.

People have used coke bottles, milk bottles and other temporary fuel containers - even condoms - as short term range extenders for decades. They are just that though - temporary. You fill them and as soon as you've got some space in your main tank you pour it in. You shouldn't really use them for more than a few hours (less for condoms!). I've not had any problems with chemicals from the plastic affecting the engine but as coke type plastic does go hard after a while with petrol there's probably something being dissolved. It takes a while though and for very short term if the bottle doesn't dissolve as soon as put some fuel into it I'd have thought it would be ok for an few hours. All of this is really only needed in some more remote parts of the world. In Europe you can buy 3L and 5L plastic fuel cans for a few euros that will hold petrol safely for years.