As to the question of who can play guard: the first thing is that my reputation as to being able to answer such questions is overrated, so caveat emptor.
2). "Traditionally" the guards are slightly shorter than the tackles and this was true mainly because more "shifting about" was required [sometimes just "pulling" and sometimes to pick up originally messed up blitz protection] and shorter legs tend to allow lower postures and less entanglements [including with the QBs and RBs].
3). Also traditionally you had the concept of the "strongside guard" who was a rhino and not necessarily much else. Massive specimens like Chris Stewart fit that mold but not necessarily where Kelly is taking this team. Alabama tends to still do this, and I think Stoops at Oklahoma would like to also if he had the monsters. But Kelly does not seem to be going this way.
4). So what's Kelly doing? We've seen some of that, but it feels to me to be a moving target this year. Naturally Kelly wants O-Linemen who can both block with power AND also move their feet. His spread emphasizes pass-blocking modes and so his linemen MUST have good feet as the number one priority. Guards however still pull more than tackles [Centers pull too --- Cave surprisingly often and effectively] and we will still probably see some traditional tackle-taller-than-guard phenomenon: Note that Chris Watt is one of our shorter linemen, and Lombard at 6'5" is not a skyscraper.
5). But Kelly is firmly in the 21st century and the day of the "Fat Man" is over. Although all of our current verbals are 6'6" minimum, none, not even the 6'9" McGlinchey are yet over 300 lbs.... yes they will grow, but these are not blubbery individuals in any measure. Also note that they ARE 6'6" minimum. Kelly/Hiestand are going tall. Why?? Ask them; you'll get a better answer; but I believe that it is because in the Kelly spread the qualities of good-feet-in-space tend to encompass all the line positions and begin to make them more equivalent... yes, they're still different but they become closer to being the same guy.
6). Now for part of the "moving target": this year Coach apparently is going to be a little different and part of that will involve lineplay. What?:The two TE alignment with one of the two being mainly an in-line blocker [translation a highly-mobile extra tackle]. That sounds like an increase of power drive blocking and internal pulling perhaps even by tackles more often. There will then be a requirement for "everybody" to "pass strike and regroup footwork", drive block, and pull at least occasionally. We won't know until we see whether Kelly is strongly emphasizing certain aspects of these packages or if "new stuff" just shows up rarely as a surprise. I think that Niklas to TE is telling us that if you want to play in the 2012 version of Kelly's hybrid "try to guess what we're doing" spread, you better be "everyman" as a lineman. That means: good to great striking levers, really good feet, ability to drive block, and quickness to pull. Sounds like a recipe for producing NFL linemen ---- now, just what did Hiestand do at Illinois???
So, what's the answer to the original question? All these guys are tackle-tall; so who's got the best quickness and agility?? [by quickness and agility I mean more than great balance of foot position and quickness that way; I mean ability to rapidly change the direction that your 300 pound mass is heading]. That's who the likely guards seem to be. I believe that two of the still outstanding O-line prospects are also D-line prospects --- maybe guesses could start there.
