I am now using a Golden Gate pick on my thumb. (I started with Dunlop, but like the Golden Gate better). I have both Dunlop L and XL, I like the L better.
There is no one right way to wear picks.
"They" say that 90 percent of players are using either Dunlop or National.
Expensive picks are a complete waste of money for a beginner.
I am intrigued by the ProPik heritage (at $14 each from Deering).
You can buy a tube of 20 Dunlop 0.20 Nickel silver picks for $25 on Amazon.
"Blue Chip" makes some highly regarded and expensive thumb picks ($40) that are metal around the thumb with a plastic pick blade. They are comfortable and durable. People say a $40 thumb pick is a complete waste for a beginner. Maybe. I am curious to see how long the blade is.
They say Earl Scruggs shortened his thumbpick, so doing so is certainly an option. If the pick is constantly banging the head, cut it down. If you feel like doing so, cut it down.
I started with a Dunlop size L thumb pick. People say these are the worst of all thumb picks and indeed are too long.
Golden Gate thumb picks are recommended. You can go with "clown barf" (bright multicolored) or as I just did, a 4 pack of ivoroid from Amazon for $12. These are recommended and not as long as the Dunlops. Mine are labeled "GP-10" made in Japan, "Golden Gate GP-10-4PK Grained Ivoroid Thumb Picks - Large - 4 Pack"