do not get "some" stars - they require a LOT of sand each - so a 75 gallon is actually kind of small for even one sand-sifting star - I've heard the figure 10 square feet PER star - and you probably don't have that much.
I have one SSS in our 72 bow and he's doing fine - we've had him for about 9 months - but I made sure I bought a SMALL one. Same with the tiger tail cucumber - I have a REALLY small one, that has doubled in size in about 5 months, and I'm getting worried that he may outgrow the supply of detritus (we only have 6 fish).
Also - we have on in our 30 gallon long reef at home - and this is the one that REALLY has too little sand to survive according to the available data - but again, I bought him really small, and he's 11 months in the tank now.
Conchs, I've never been able to keep alive - I won't buy any more for a long time.
I would recommend an emerald crab for cleaning the rocks, and I'd get a couple more different types of snail - and less of the ones you mentioned. My preference is to have fewer examples of each, but as diverse a cleaner crew as possible.
For instance - my 72 bow has:
2 zebra hermits - these grew too large in the 30
8 blue legged hermit
4 scarlet hermits (these did not survive)
10 astreas (down to 6-7 now)
6 turbos
1 sand sifting star (small)
1 tiger tail cucumber (small)
10 limpet snails (came with live rock, and breed like crazy)
4 nerite snails (down to 1)
3 cleaner shrimp
1 emerald crab
I am about due for a top-up, and I'll probably get just half a dozen of each of my preferred snails and hermit and put a couple of each in the 30 and the rest in the 72... when I would have my cleaner crew too biased towards one type, say blue legged hermits, then I found that the attrition rate was increased significantly. Now I have diversified, I find they live longer - imagine that eh?
hth
Greg