3 guppies and a couple of Otocinclus would be a great choice for your tank. You can add whichever fish you want first. Honestly I would tend to just buy all 5 of them and put them in at the same time. Just make sure that you do weekly 50% water changes for the first few weeks. If patience is your friend it might be better to buy either the guppies or the Otocinclus first. Then wait a couple of weeks and add the other fish.
As for the rocks it can be fun looking for just the right ones. Some rocks will affect your water chemistry though. The wrong types of rock can cause your pH to rise as well as increase your general hardness. The good news is that you can easily test for this. All you need is some kind of a strong acid. Hydrochloric acid works. Just place a drop of it on the rock you want to test and if there's no reaction (bubbling) then you're all good.
You're right, Crystal Red Shrimps aren't legal in Hawaii. The Hawaiian ones that you mentioned actually appreciate some salt in their water so they aren't the best for a planted tank. The regular feeder shrimps will work. Not as pretty though.
I would guess that there are definitely hobbyists on Maui keeping and breeding CRS. I'm not sure if there's a fish club on Maui so it might be a little hard to meet other hobbyists there but try asking around at the different pet shops and you might eventually be able to find someone who has some.
Good Luck. If you have more questions you can PM me if you want. I don't tend to check the Hawaii section of the forum too often.