Are there any particular types of sand or anything that you can use to top off the MTS, or just any sort of inert sand works?
If you read through this Sub-Forum it seems other members have had more problem with fine gravel Caps than Sand Caps.
Any inert sand will work. Most need a good rinse, some need more rinsing than others. Other than rinsing it's really a matter of color preference.
PFS - Pool Filter Sand is Snow white (Not NYC Snow white)
Play Sand - Yellow
Black Diamond Blasting grit - Black
This three are very uniform.
For a mist of fine sand and very small pebbles there's Sakrete All Purpose Sand also yellow.
I've used all but the PFS. They all work. Currently I'm using a mix of Play Sand and All-Purpose sand. I feel the really benefit is the very thin "seal" layer. the finest sand will settle on the top of the MTS(Dirt). I believe this holds the nutrients or slows down their movement up into the water column so the roots of the plants can get them. Also preventing over feeding of the water column that would stimulate Algae growth.
But, I can't prove that theory