I can share with you that we (my team of 4 warlocks and 1 priest) killed the prince in karazhan yesterday.

This was done with my gf and 4 other guys from my guild.

10man raid should not be too difficult if you have some online friends since you litteraly are half the raid.

25-man however could be a bit more difficult, but might be possible if you have an open minded guild. However I must say I cant blame the guild if they dont want you in their raid. Playing 5 chars at once will allmost certain gimp your performance in some way, so its usually just better to bring 5 different people for those raid.

Personally I might aim for using one character as a main and bring the others in as reserves when we're lacking people, people are slacking, dont show up at the right time, etc.