If not, when do I use which?
Are „Hänsel and Gretel“ any different from „Hänschen and Gretchen“?
Is a „Mädl“ smaller / younger than a „Mädchen“?
-l as a diminutive suffix is typical for southern Germany, your example "Mäd(e)l" for "Mädchen" is one of the rare cases where this form is also common in Northern Germany. So, -l always has this bavarian connotation, rathern than implying a degree of diminuition. In the case for Mädel and Mädchen, both are etymologically diminutives but neither of them aren't perceived diminutive anymore, since they became detached from their original main form ("Magd"/"Maid").
The diminutive suffixes -l/-el/-le/-li are southern dialect variants of Standard German -lein.
The diminutive suffixes -je/-tje/-ke are northern dialect variants of Standard German -chen.
Both -lein and -chen are used in Standard German but often one is much more common than the other for a certain noun. For example Mädchen is much more common than Mädel, at least in Standard German.
But that's only for Standard German. Names and typical dialect words for everyday items feature the diminutive suffix preferred by the dialect in that area.
On top of that, diminutives may be frozen, as for example in nouns as Mädchen or Mädel which is not perceived as a little maid any more, or in names as Mareike which is not perceived as little Mary any more.
My impression is Mädchen refers to younger girls, at most 8 or 10 years. Meanwhile Mädel refers to tweens (10-12) and teens. Adult women sometimes refer to themselves collectively as Mädel as in Mädelsabend="girls night out". But this only applies to the root Magd. In general, while you many be able to deduce an approximate meaning by recognizing the suffix, you have to learn the exact meaning and connotation individually; word meanings and usage tend to drift over time. When it comes to names I gather it's more according to region than anything else. There are names "Hänschen" and "Gretchen" and I doubt the story would be very different if those names were used. (The name Gretchen has spread to the English speaking world and is actually fairly common. Not Greta or Gretel though. There is a saying: "Was Hänschen nicht lernt, lernt Hans nimmermehr." I don't know if it works with Hänsel or not.)
It is, as usual, complicated. In English before 1970 the word "girl" could refer to a woman in her late twenties. After 1970 the custom changed, but only for that one word; the meanings of "puppy" and "kitten" didn't change at that time.