How many in a run is a guild run? consider for example MC, is soloable, currently guild credit is awarded for 80% participation, does this mean you have to bring at least 32 people to get a guild achievement for a raid which is soloable? This question pretty much applies to all 5 mans, and raids that are stompable with much smaller sizes. what is the threshold of guild members required to count toward a guild achievement?
In general we require 80% of the group to be comprised of guild members. There are exceptions to this case though like 40 man raids and arenas. All the gritty details below are below.

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How many in a run is a guild run? consider for example MC, is soloable, currently guild credit is awarded for 80% participation, does this mean you have to bring at least 32 people to get a guild achievement for a raid which is soloable? This question pretty much applies to all 5 mans, and raids that are stompable with much smaller sizes. what is the threshold of guild members required to count toward a guild achievement?
4/5 - dungeons
8/10 - 10 player raid, 10 player BG
20/25 - 25 player raid, 25 player BG
10/40 - 40 player raid
12/15 - 15 player BG
2/2, 3/3, 5/5 - Arenas
40 player battlegrounds will be converted to 25 player for rated matches. Guild achievements are only available in rated BG matches.
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20/25 - 25 player raid will be kind of hard to accomplish for the small guilds out there.
If they can bring up 20 pple for OLD content, they surely can take 25 pple for NEW content.

For what I understand, you said you wanted to give both type of guild an even start.

10 players raid guild will not have that kind of recources.