Can a guard take a snap from the center if the QB or RB doesn't? Is the guard an eligible running back?
The answer is no. A guard cannot take the snap directly from the center.
Anyone not on the line of scrimmage can take a snap - rules of football. A guard is a position next to the center on the line of scrimmage so the answer is - No.
A guard however can receive a handoff (not a snap) from anyone including the quarterback. The guard can also pick the ball up off of the ground if the center chooses to leave it there (many trick play variations here).
Article 4: Legal Snap. A snap is a backward pass. The snap must be received by a player who is not on the line at the snap, unless the ball first strikes the ground. If the ball first strikes the ground, or is muffed by an eligible backfield receiver, or quarterback under center, it can be recovered and advanced by any player.
If you are asking whether a player that typically plays guard can take the snap, the answer is yes... with a real caveat:
From the NFL rules...
First, Rule 5, Section 1, Article 2:
PLAYERS NUMBERED BY POSITION
All players must wear numerals on their jerseys in accordance with Rule 5, Section 4, Article 3(c).
Such numerals must be by playing position, as follows:
(a) quarterbacks, punters, and placekickers: 1-19;
(d) offensive guards and tackles: 60-79;
But second, Rule 5, Section 3, Article 1:
REPORTING CHANGE OF POSITION
An offensive player wearing the number of an ineligible pass receiver (50-79 and 90-99) is permitted to line up in the position of an eligible pass receiver (1-49 and 80-89), and an offensive player wearing the number of an eligible pass receiver is permitted to line up in the position of an ineligible pass receiver, provided that he immediately reports the change in his eligibility status to the Referee, who will inform the defensive team.
So, combined with Coach-D's answer on how a snap must be performed, I believe this narrows it down that a person who usually plays as guard can take the snap... but only after they report as eligible (since all backfield players are eligible, and you must be in the backfield to take the snap).
That player also cannot return to playing guard until they skip a play (or a timeout, possession change or similar events).
I would think this analogous to the Wildcat formation, in that:
- The ball is snapped to someone who isn't traditionally the quarterback
- Five players must report as eligible receivers should the not-a-quarterback choose to throw the ball
- The player's initial position isn't relevant, so long as all other positions properly report
A guard that is declared as a guard likely can't suddenly line up as the quarterback, and if they could suddenly shift the formation, it wouldn't be wise; all of a sudden one's very important quarterback is now on the line of scrimmage, staring down several men who are more than capable of running right through them.