These days no1 plays Sealmaster in existing DS guilds. Simply Crusaders and Clerics are the best choice for DS. If you are rich then go for Crusader if you are poor then even with lv 35 uns eq Cleric will be very good and usefull.
Having guild full of Sealmasters could work nice if only (lets say) you had like at least 4/5
skilled Sealmasters, if not - all of your seals will be recovered (since clerics in DS are pretty skilled players and like 2/3 the best ones recover sealed members in less than second). Unless you are able to seal all clerics at the same, and not die in the meantime, then idea of full SM would have chance to succeed. Although you can't forget about other players - for example Crusaders. If you don't seal Crusader you are most likely dead in 1 slash. Average Crusader in DS can easy one slash kill SM with under 6k hp.
The best DS SM I knew had around 13k hp on full con mode. He was able to kill Crus, Cleric, Voy 1 vs 1 but when situation was 2 vs 1 then he was almost always dead. Playing as Cleric / Crusader you can win vs 2 or even more players.
That is just my opinion you can try making one SM guild but I doubt it will be any good (maybe if every member can afford at least lv3 ug's and lv1 bd gems, and is pretty skilled player then I would give you small chance of success).
More sense would make the idea of making guild full of Clerics

I would love to have like 5+ Clerics every DS on my side that is instant death sentence to your opponents.