Welcome to TBN. Good folk's, lot's of advice and the price is right!
Many owner's manual's will provide a speed range for each gear - if you don't have one - your local dealer or a search on the Internet should give you a few options.
The question regarding the "best" speed for using the various implements will depend greatly on a few variables... soil condition - dry or wet. Soil type - sandy, rocky, light clay, heavy clay. Current vegetation cover - tight sod, brush, small trees, tall; heavy grass, etc.
Your tractor has more than enough power to "run-away" with the size of implements you listed.
Always take your time and go slowly... You can stop quicker and you can avoid damaging the attachment or the tractor or a nearby fence, etc. Once you're comfortable running the implement and have determined that you're able to work the ground successfully in the manner that the particular implement is designed; you can always go faster.
Gear choice will depend upon those conditions I mentioned previously as well as the type of attachment. The rototiller will take more power to run and you'll have to go slower than; say, with the disc harrow. And, I'm guessing that the plastic mulch layer attachment won't take very much "uummphh" from your tractor to run at all.. But, to successfully use that attachment - I seriously doubt that you would want to run in the highest possible gear - although; I'll bet your tractor could do it!!
Best advice - just DO IT - cautiously: until you know how each of the attachments do the job they're supposed to do and you're comfortable working with them. As long as the tractor isn't bogging down and losing rpm's or racing at the top of the tachometer scale - you're probably in the right gear...
Best of luck.
AKfish
