So you are the DM and out of a five party group four Elves have extremely high Acrobatics skills. They want to use their skills as a team. (E.G. one setting themself low in a low defensive crouch allowing his comrades to run and launch themselves up and over an enemy defensive line. E.G. grabbing a comrade running parallel to the defensive line and redirecting them perpendicular towards the line in a low tumble through the line. E.G. Creating a standing human pyramid to sneak over the castle wall at night.)

When out of combat, set a DC, ask the group for their Acrobatics check, if half or more succeed then it is achieved, if not the attempt fails. In combat will require a lot of Readying actions, being aware they cannot ready bonus actions or movement (except as a readied Dash action). 

Also, I think you mean E.G. (exempli gratis = "for example") rather than I.E. (id est = "that is"). 

I expect this team will make extensive use of the "Help" action, both in and out of combat.

I would strongly recommend sitting down, out of game, with the Elf players and carefully defining exactly what Acrobatics can and cannot do. What the DCs are. What the consequences of failure are, etc.  This is a set of situations and skill uses that are not defined in the game. Best to work out an agreement on exactly what can and cannot be done before it becomes an issue in play.  The description of an Acrobatics (Dexterity) check in the PHB pretty much puts it at DM's option whether any of these proposed stunts can be attempted.

I’d definitely let them try. It will take a little bit of prep work to figure out how and I’d make them do all of the prep work except for actually setting the DC’s of the checks, but they’ll have fun doing it so let them.

As I did not make it clear in my post above, I support the Elves in their clever use of Acrobatics.  This is the sort of stuff I love players doing. I advocate for working all the details out in advance because it makes the actual game play much smoother and more fun.  Things don't have to Coe to a halt while everybody calculates DC and modifiers. the DM doesn't upset the players by just saying NO. The players don't upset the DM by pulling a completely unexpected stunt. Everybody can have fun with the characters doing awesome stuff.  If this proposed use of Acrobatic is not going to work at all I would argue that the players would be allowed to rework their characters, as this seems to be something pretty central to the character concepts.

Thank you for all the responses! As you can tell they are wanting to have that Legolas open combat with the mobile feat in mind. I was just trying to get good ideas to judicate as close to the rules without inpinging on on the fun of the group.