All high energy and high power density battery chemistries are dangerous by definition. Do not mix batteries of different sizes. Do not mix different battery chemistries of any type.
Even cells of the same chemistry, size, and brand cannot be connected together in parallel before carefully matching their voltages or else they will try and charge each other, and for batteries that need careful charging (like lithium batteries) this is dangerous.
You've heard of smartphone battery fires, right? That's what happens when you do not charge lithium polymer properly or mis-used. Radio-controlled models use particularly large versions of the same battery and they are extra careful and use expensive chargers. Models, homes, and lives have been lost from mischarged and over-discharged lithium polymer batteries. For their batteries with series connected cells, the battery chargers basically monitor and charge each cell individually so it does not differ from the other cells. They are not blindly charged as a group.
Also, a 18650 lithium battery is not the same as a 3V coin cell lithium battery, not even in chemistry. That's almost like calling both NiCd and NiMh nickel batteries. There are at least several different kind of "lithium" batteries out there. If all you know about a battery is that it is "lithium" then you don't know anything about the battery. There's Lithium ion, lithium polymer, lithium iron phosphate, and whatever crappy non-rechargeable AA and AAA batteries labelled Lithium are. They're all different.