I think snails are a pretty good idea, but they can be tricky. We had a small 10 gallon tank especially for some snails and it got over run pretty fast. They were apple snails and so I am not familiar with most other snails except for what I have read on here.
However, I do know a little about the eels you mentioned. I have two peacock eels and yes, they are really good at burying themselves and would most likely be ok at 'churning' a sand substrate. I did notice though that you have quite a few 'bottom dweling' species and this could cause the eels to stay in one spot and not dig around much.
I bought my eels not only because I thought they were awesome, which they are, but also to 'pick up' after the other fish. This worked great for a while, but then I noticed them getting skinny because as my other fish got bigger, there was less and less food getting to the bottom of the tank. The problem then became trying to get food to the bottom and I was forced to over feed the tank until I could get a tank solely for the eels.
So, while I am not sure if eels would be the best option for your tank and the purposes you need them for, I hope I could help point out some of the problems that eels may cause. Also, you will have to consider your tank size, as usual.