Tangs for less than 100G (and only do one):
Kole Tang
Tomini Tang
Convict Tang
Scopus Tang
Yellow Tang
If you're going to do a tang in a 75G tank, keep the stock list light. IMO, it's not worth it, you can get many more small colorful fish that will add much more "life" to the tank than one large fish such as a Tang.
Sand sifting starfish:
I have two in my 155G. There is a lot of controversy surrounding whether or not they belong in tankgs because they very often starve to death. I have had these two in two different tanks and have been very lucky. I have a very healthy sand bed, so they're not going to starve in my tank. They don't belong in a tank that has new sand (less than a year old). Many people claim that they eat all the good fauna out of the sand bed, and this can be true, especially in a small tank or a shallow sand bed. In most cases, I do not recommend people putting the in their tanks, but I will say this, they keep my sand clean!
BiColor Blennies:
fun little fish and IMO a great addition. Easy to keep as well.