Present Arms swaps your library with another deck you own from outside the game and use that one instead ("library" and "deck" both refer to the same thing, but the former is used to refer to it in-game, whereas the latter is used to refer to it outside of the game)
A deck can have any number of cards named Relentless Rats
The -2 ability also has good synergy with his +1 effect however this only comes into play if Sorin has been in play for a little while so as to build up a decent count of lifelinking tokens
TLDR A traditional Magic: The Gathering card is about 2.5 in 3.5 in (roughly 6.3 cm 8.8 cm)
You can help remedy this by practicing with the deck, goldfishing, and generally getting more familiar with how the deck plays and what it wants to accomplish
But the more Ive thought about it, the more I like it, and I think Im going to try it out in a number of my red decks