10mans are great for a 25 progression guild.
Because of the smaller format, you can grab a bunch of people with more gear than the raid is tuned for and learn the fights coming up in 25s. I cannot stress how invaluable that experience is in making sure every moment in your 25 man run counts. I have run a lot of 25s, and as a raid leader and as a raid member, it becomes very obvious very quickly who has and who has not run a 10 prior to the main raid. I have also noticed it very distinctly in my own ability to adapt to the craziness of 25s if I have had a crack in 10s beforehand.
10s are easier to put together, you can do them in downtime and have fun with your little group of friends in the guild.
But what happens when a group of people in the guild form a 10man and they start progressing, and they kill the endboss and start on hardmodes while another group is struggling to get people together? What happens when the group of people are all the officers? What happens if the officers are split between 2 competetive groups unevenly and they end up having fights in officers down those lines over 25man content and stuff "for the good of the guild"?
In short, 10mans can be a very divisive situation in your guild. They necessarily isolate people into little groups and those little groups form very tight knit circles of friends. This in and of itself is not a bad thing. Circles of people will happen wherever you go. What makes this situation different is that new people will find it hard to join in, they may feel alienated and unwelcome on top of feeling unsure in a new guild. This, of course, probably isn't the case, but how you spend you non raiding time is really important. It should be encouraged for people to mix and form friendships understandings and trust so that their 25man behaviour is more accommodating and overall raid performance improves.
How often have you thought so and so was an idiot only to find out maybe 2 months later, that, no they are not? And the only way you found that out was by running chain heroics with them one day because you had a spot free and they came along? All this time you thought they were an elitist snob because they were the RL of that 10man that ran on sundays and never included anyone new. After that, you may have trusted them more in raids to do their job, or when they said that blah blah happened instead of thinking the worst, you might give them the benefit of the doubt. Then maybe a week or two later, you start enjoying raiding again and your OWN performance improves.
Don't underestimate the impact that fracturing your guild members can have on your morale, and therefore, raid performance. Not everyone has a robust personality and wont take being told "no, we dont need blah class for our run" to heart or even more unintentionally hurtfully hear "We worked for weeks to get this group together, and we enjoy running together, we don't have room for everyone". Ouch. Yeah, I head that said recently in my guild. While very true and factual, it was really very upsetting to the poor person on the receiving end (and no, it wasn't me).
Lastly, 10s should always take a second place to 25s in a 25 progression guild. If the officers need another 25 night for a recruitment alt run, and the best time they can find is at the same time as your 10man, don't emo out at everyone saying how unfair it all is. Talk to your officers about the situation! It might be that the 25 is completely optional and you can continue your 10man while another alt 25 is running. But at the end of the day, if the 25 run is "official" (ie a recruitment run offering all the perks of a normal guild run, or a new progression night) then, no matter how put out you feel, you are there to raid 25s, not 10s and you should support the guild in that endeavour. If you are questioning this, mayhaps this guild isn't for you after all. And you know what, that's ok too.
As officers and guild leaders, you should be sensitive to all the things I have spoken about above. Be aware of which officers are attending which 10s. Try and find out from those not attending 10s how they feel about the situation, how they have been treated. Make damn sure that no divisions spring up within your guild based on who goes to what 10man. If you can, as officers, mix up where you attend. Be "busy" one week for your regular 10 with your officer friends and see if you can sit in on another one. You might not think it immediately, but morale and raid performance is greatly affected by this out of raid stuff, because no matter what happens, people are people and you need people to defeat the game and earn purple (and sometimes orange) pixels!