Jake
410 microlight is a thickener without microballoons to give a really lightweight filler.
It spooky stuff, it almost binds together when squeezed in the hand BEFORE any resin is added. So it needs very little resin in the mix.
It actually floats when cured.

I think that it is worth the money and better to keep with one manufacturer when doing a big project. Any advice or warranty would be void if you start mixing and matching.

Either way, I can't believe how much filler Man-shed is using on his foam, you only need to fill the joints because filling and fairing will be necessary after glassing anyhow.