Ok, so you got you some people to play WoW with. So let's look at healers as an overall section.
Pallies (Take 2)
These guys are your staple MHs. They are the best bar none at MT healing. They now have a bunch of "OH SHIT" buttons to deal with spike damage and quite a few more insta casts to deal with it. They can bubble themselves out of trouble when everything goes to poo and chain cast massive heals in a VERY fast time when they are specced correctly and judging correctly.
Droods (Take 0-2)
These guys used to be MH as well, being the HOT component of MT healing. Nowadays they have been buffed disgustingly and can roll hots on a tank as well as AoE heal. Trees are, however, probably the first class I would drop as all their abilities can now be easily covered off by the 3 other classes. A good druid is still a great asset to any healing team.
Priests (Take 2-3)
These guys really are starting to some into their own. They are the swiss army knives of healing and I would NEVER go into a raid without a Holy priest. My next choice would be a full disc priest. They have 2 very seperate roles.
Holy priests are OHers. They spam the CRAP out of CoH and double the next non holy priest on the healing meters and if they aren't then something is wrong (except on specific fights when MHers shine). This will be getting a nerf soon, but I dont see that it will stop holy priests being top of the healing meters, just that other healers will have to do some actual healing work.
Disc Priests are very difficult to measure their performance. They are MHs and compliment a holy pally BEAUTIFULLY in their design. Because Disc priests MITIGATE damage with thier bubbles, there is no way to measure that as yet. They will be on the bottom of meters, but that is not a very accurate measure of their performance. Be careful with a disc priest, as they can potentially start to starve MTs of rage/manabyhealing if they are over geared for the content they are doing. A good Disc priest will know this and compensate.
Shammies (take 1-2)
TOTEMS FTW! These guys bring totems and chain heals. They are more utility healers than anything else and seem to be middling in terms of raw healing output. But the bonuses they bring to the other healers are not to be sniffed at. Also, every boss drops resto shammie shit. Bring one as the garbage collector. Will invariably be the best geared toon in the entire guild!
So there's a high level summary. You should be aiming for 7 healers, but this can +/- 1 depending on content and your ability of your healers.