@SaturnX,
Thats a few questions there. I'll summarise my experience. I have yet to upgrade to an SSD but am about to. The conclusion is, get a 1TB SSD if you can afford it and leave the optical bay alone. Otherwise, read on:
On the efi firmware issue with this macbook:
If you're on Snow Leopard, everything works well with efi 1.6 (and 1.7 i believe).
If you're on Lion or anything above that, you should downgrade to efi 1.6 (as 1.7 has issues).
However, on the latter point versions of Mavericks (can't remember which one), you begin to have issues with 1.6 again (and need to upgrade back to 1.7). So if you're on the latest Mavericks, 1.7 it is.
I don't know about Yosemite but I expect it will be the same as Mavericks.
In summary, you should have both installers somewhere, in case an update changes things and need to upgrade/downgrade. Also notice that this is primarily a hardware issue that Apple never acknowledged, so different macbook pro mid-2009 (even though they are exactly the same, might behave differently due to not having the exact same component).
As for the boot SSD. I would prefer to put it in the optical bay, but there are issues with that (something to do with safe sleep and the scratch disk), so you're better of putting your boot drive in the main hd bay. Even if those issues don't bother you (you may be able to get around them), you should still do it all the same because:
1. You might accidentally eject your boot drive by pressing the eject button on the keyboard (not a good thing).
2. You can take advantage of the eject button to eject the old HD when you need more battery whilst on the go. Most people will not miss the dvd drive, but they will miss the long battery life. The eject button takes care of that. You can easily remount it using disk utility, making a script, terminal alias, etc...
As for the type of SSD, crucial and samsung come recommended. As for the size: With modern SSDs you should go for at least 256GB, with older SSDs you can go down to 128GB. This is because the SSD speed for sequential write changes (from 100 to 500 mb/s) as their size changes. Older SSDs tend to have better sequential write speeds for 128GB. Note however that it is the random write speeds that you will benefit from.
Having said all that, your best bet is to just go for the cheapest SSD you can get your hands on and, depending on your use, 256GB. A light user might be happy with less, a heavy user will need the extra space (primarily to use as a scratch disk or cache for 2D/3D/video editing but also development (server cache, etc...).
As for whether you really need one. I'd say that if you're a happy user of Snow Leopard or Mountain Lion (you should not use Lion), than you probably don't need an SSD. If you're using Mavericks or above, you will start to notice a need. Modern OSs are starting to write to disk much more, now that SSDs are becoming commonplace.
i prefer samsung ssd