1) That's okay, I think.
2) Yes, I think so. No need to have both SBPro/SB16, except for demoscene stuff (SB Pro).
3) No real OPL required (except if you need authencity). Some Windows games used OPL music, too. (Warpath, Dominate!) PnP SBs can disable their OPL.
4) Few. If you like to use MT32/GM emulation, you *may* need a bit of additional RAM (say 1 or 2MB).
5) Yes. But driver versions could be different, don't know. Also, the SB32 is a cut-down AWE32 (limited effects).
6) Yes. They used it for high quality mixing on lower end hardware.

AEV80 wrote:

1. For Adlib and SBPro games I was planning to use a Yamaha Audician 32.

Good choice.

2. Is the AWE32/AWE64 backwards compatible with all SB16 games? Do i really need to get a SB16 and if I do what''s a good clone (low noise) to get?

Yes it is backwards compatible.
No you don't need a SB16 for DOS games because it's shit (in my honest personal opinion), SBPro is all you need.

3. Are there any games that use OPL music with 16bit effects? Do I need a real OPL on a SB16 or AWE32 unless it's for playing games that came out pre SB16.

DOS games? None, Zero Zip, Null.
You don't need a SB16.

6. Are there many DOS games that actually used the features of the Gravis Ultrasound?

Not many.
Go for a Sound Blaster Pro or a clone.

But there are LOT's of Demos that require a GUS, therefore a GUS and an AUDICIAN32 is a really good choice.

Thanks for the advice guys.

I'm feeling like a stuck record, but there's a few extra clarifications I'll add to what James-F said:
1) A few early Apogee games (Duke Nukem 2 and Major Stryker) used certain ADPCM sound modes for some effects that are not supported by Yamaha sound cards.
2) SB16 or AWE is by far the best way to play games using Asylum sound, particularly Crusader No Remorse and Crusader No Regret.

If neither of these things matter to you, then his advice stands.