Hey there bash-tastic.
When do you anticipate making the dedicated 10man group? Or were you just going to try to make it once we don't have enough for the 25? If we are going to seriously try to do this soon enough to pass around the heroic only mount, we need to form this ASAP. WoD comes out on 11/13/14, leaving us with 12 more raid resets, counting this week. The only way we'll get a dedicated group their mounts is if we start working on it THIS week and somehow have heroic garrosh down in 3 weeks, which seems a bit far-fetched.
Edit: Also, where does this leave us for recruiting? We need to make this decision soon because it affects recruiting. We cannot continue to recruit people if we know that our 25-man will be taking a break soon.
When we did this for DS, I think it was just me and Thundere raid leading the group, so I remember what we did, what worked and what didn't. We did indeed kill heroic Deathwing before the release of MoP. As a matter of fact, we killed him 9 times and got everybody but one person (Of our dedicated 10) their mount! We had a dedicated group of 10 people and an additional 2-3 people that we would bring in occasionally for easier bosses but we stuck wtih our 10 for progression. Those 10 people were all excellent raiders who were on every day for raid and there was no drama. We had 2 dedicated healers (Me and the beast that we call Jlogg) and one hybrid healer (Thundere) who was able to DPS or Heal competitively depending on what was needed. I'm not sure if we'll need the same setup for SoO or not. I cannot stress the importance of the 10 people we had in for progression all being 100% attendance. Unless you have 90%+ attendance, you're not going to get into the 10man group.
Regarding WoD raiding: All lockouts are separate now. LFR, "Normal" (Old Flex), "Heroic" (Old normal) and "Mythic" (Old heroic) are all on separate lockouts. If we have 30 people who want to raid, we could conceivably full clear heroic with 30 people then bring the best 20 for mythic progression. We will be able to be flexible here.