Assuming you pick a decent weather window, you wont have any problems. I would suggest you transit via the Wolf Rock Lighthouse and go into the islands via the southern entrance to St. Marys. The north entrance requires care especially at low tide.
When cruising the islands beware area around Maiden Bower. Have found several uncharted large rocks just below surface-difiicult to transit in heavy Atlantic swell.
Harbour Master is very friendly. Arrange you fuel IN ADVANCE from Dave Sibley, and the collection time of your fuel (it will be in cans). See also thread "Trip to the Scilly Isles" on this forum.
Mooring is via bouys in sheltered harbour. If placed by the HM close inshore, remember to raise you outboard leg (tidal harbour only stays wet 75yds from shore). Transit to the harbour wall in by thumb and smile, so drop everybody/everything on the mole before bouying up.
Accomodation is limited and expensive and MUST be arranged in advance.
The islands are georgous and well worth visiting despite any negatism you may infer from the above. Landing/camping on many islands forbidden or restricted. CHECK FIRST.
Contact me direct if you need any more info. When are you going? A bunch of us go down regularly (I am in Torquay).
Dont recommend you take 8 on 6m boat. Suggest limit to 4.