1) If you're talking about the M33 firmware, then yes, 5.00M33 is the latest. But the most recent patch for it is 5.00M33-6. If you're talking about custom firmwares in general, there are newer firmwares like 5.50GEN and MHU firmwares. 5.00M33-6 is more than good though

Also, there's no such thing as the "max" point for custom firmwares/official firmwares unless Sony stops developing firmwares. It'll keep growing till the point Sony gets fed up lol

2) Um, buying an original stick wouldn't really be an option as it would be pretty darn expensive. Really unreasonable. You can buy a fake stick, but when you use fake sticks, chances of it getting corrupted is pretty darn high. Also, you may experience a couple of lags when browsing through the contents of your memory stick in a system. IE : PSP, PC etc.
3) Manual, go to the Playstation official website.
4) No idea, not a slim user here.

5) Pouch, really you ought to check them out and see which one suits you best. Some may like big and bulky ones that can store lots of stuff like UMDs(with covers) etc, whereas others may prefer small and sleek looking ones just to keep a UMD or two and maybe a memory stick. It's all up to you.
Wah very fast reply....thank you, at least now I have some knowledge on what to look for.....