Anything stopping me from setting it up where 1 item drops 90% of the time but a better item drops 10% of the time?
Nothing will stop you from doing that.
I could add as many items as I wished, I would just need to string it out quite a bit and keep the summed value of drop % to 100%. so random drop of 20 different items would have drop chance of 5% each?
You could do that, but you should understand that it won't be weighted the same as a random table roll. For example, this is what a table might look like:
01-20 - Item 1
21-40 - Item 2
41-60 - Item 3
61-80 - Item 4
81-100 - Item 5
But, in FRUA, it would look like this:
20% chance - Item 1
20% chance - Item 2
20% chance - Item 3
20% chance - Item 4
Default Item - Item 5
You will tend to see Item 5 more frequently, versus an equal distribution, due to the fact that each node has an 80% chance of failure. So, keep that in mind when planning it all out.