Journalism in this country - spew! Pyke?, Tomkins? for Olympic 7's? This would be funny if they were joking but the fact that they are being serious is just disturbing. I watched Tomkins in that BaaBaas game & the try was the one good thing he did.
IMHO Olympic 7's (in addition to going for Gold) is a promotional tool to get more people into following & playing Rugby with the hope that it will flow over to traditional 15's. Allowing players straight into the Olympic 7's team from Loig defeats that whole purpose.
I think players can struggle going to or from 7's or 15's depending on their genes, build, playing position, etc. For instance, 80min (well, 75min in women's club Rugby) of being bashed from pillar to post & returning the favour as a flanker in 15's exhausts me a lot more than playing x2 7's games back to back. I'd summarise it as 7's requires more "lactate acid tolerance" which you need in 15's too but the onset is sooner in 7's due to the lack of rest (which you usually get in 15's come set piece time) between intensive sprints.
15's has what I'd call "Contact endurance" more tackles, more people in the tackle & the break down, less gaps leading to more contact, more hitting the deck & having to keep getting back up (as 7's tend to try & avoid going to ground), more strength related pushing activities such as rucks, mauls, scrums, etc (yes 7's has these but there are far less bodies/force to overcome & less emphasis on them). In short you could also say it is "Muscular endurance related". It is a different kind of exhaustion.
Despite what I have said above on the differences I don't believe it is out of the realm that hybrid players can exist. As far as 15's flankers having a go at 7's is concerned I like the idea of flankers who are capable of playing as backs - such as Colby Fainga'a.