That's correct, Mods such as the penny have a firing pin much like that of a regulated mod, this allows for the positive of the battery to make full contact with the positive firing pin on the Tank or RDA you are using. The Apollo for example, wont allow this because of the fact that the firing pin on the Tank physically touches the battery and because the Positive and Negative pins sit flush together it will cause a short. Usually the firing pin and negative "wall" surrounding the pin sit flush together on tanks where they are usually adjustable on RDA's
I will find an example for you to understand this better.
Here are 2 examples I drew up to quickly give you an idea:
As you can see the firing pin on the RDA protrudes out the bottom of the RDA away from the "Side Wall" The side wall acts as the "negative"
As you can see the firing pin on the Tank is hidden, allowing the positive of the battery to make contact with both the "Side Wall" (negative) and "Pin" (Positive) resulting on a short.
I hope this helps.
With Regards to it getting warm, A mech usually heats up from discharging too quick. It will get too hot if you vape too "hard" on too low of a build or if you fire for too long. On a penny mod and a general Samsung Pink or Blue battery I would go lowest around 0.4 ohm and on anything like a vtc 4 or Efest purple 35A as low as 0.2 with drags of up to no longer than 5 seconds. This has proved to be safest for me at least.
The penny is known for heating up from time to time, just make sure the positive post of the battery isn't venting (sounds like air is escaping) and make sure the battery inst hot to the touch.