You see my guild is a 10 man heroic guild and we are looking to go to the next xpac and we a mythic raiding guild .
so what i want to ask you is should we change to 25 man and raid to try to gear people before the xpac or stay in our 10 man and push for heroics and after that then find the other 10 or so people we need ?
You can recruit like crazy right now and be in a weird position with a large bench on farm content (assuming you're 14/14H if you're already worrying about Mythic), then run two groups, then eventually make it into 25H before the expansion ends. This is the route my guild is taking right now. We're clearing 14/14H in two groups each week now. We just need a few more amazing ranged DPS before we can do 25H comfortably.
When patch 6.0 hits, any 10H guild that has not been actively recruiting since Blizzcon will be in a really crappy spot. They'll only be able to do Normal and Heroic (i.e. current Flex and Normal) until picking up 20 people to step into Mythic again (i.e. current Heroic). Unless you can magically double your roster with people of equal skill in a very short period of time, you probably won't be prepared in time for Mythic SoO.
If you're more of a casual guild and not already 14/14H, I would not worry too much about getting to Mythic size too quickly. If that's a goal for your guild, you'll be able to grow while still raiding normal/heroic in 6.0/WoD since it will be completely flexible. You can also jump right in with a guild merger, but those rarely work out well.
We're continuing with 10 man for the rest of the expansion. We're lightly recruiting for Mythic, not really doing much but bumping recruitment threads saying we have spots open for mythic, not guaranteeing you'll see any time this expansion. We've only got one recruit so far, but I don't blame them, who wants to sit around for several months. Once the expansion comes closer we'll start going at it harder.
I've reached out to a lot of people I used to play with, and I got about 10 people who expressed interest. Not counting on all of them to come raid with us, but if 4-6 of them do it'll be nice.
Other than that, I imagine the bulk of our recruiting will be during:
a) After 6.0. If they convert SoO to WoD format, we can start recruiting people immediately to trial in the new "Heroics" (current normals, except flexible). Once we get a decent roster, we can start doing easier Mythics. Unfortunately this pretty much means no more SoO progression if we don't clear 14/14H before then, but it's a good move for the long run.(Currently 10/14H on a 6 hour schedule)
b) After WoD's release. As a non super hardcore guild that will probably take 4-6 weeks to clear the new Heroics, we'll be able to use that time to recruit people to get up to a roster that can attempt Mythic.