the old variant of E. tennelus is tall. It is sold as E. tennelus "green". It is actually a plant by a different name entirely. Be sure not to get that one. The one I am referring to is available in tissue culture from AFA as well. Get that one. It will around the same height as the blyxxa but has a more dispersed look. Under higher light it grows shorter and gets bronzing. Neat little plant loved by Amano. Honestly, I'm not a fan of DSM. The conversion process is a PITA. You get melt and have other issues to deal with. If you can buy enough to plant densely and flood from the beginning do it. If you insist on a DSM then You could mix in the E. tennelus. You could skip it altogether if you are going to do stems behind the blyxxa. The hairgrass alone would be enough of a transition. Dry starting moss isn't too difficult. Some people chop it up and mix it in with a bit of yogurt (yes that isn't a typo) and "paint" it onto the rocks. That has to stink. I'd just chop it up and rub it onto the rocks where you want it. Fissidens and riccardia are good options here. A DSM method of use is in the photo attached. If this was me I'd skip the DSM. I haven't done a DSM since I first started. Its really not enough of an advantage for me to justify doing. The last time I did one was to get a carpet grown in so I could preserve a steep slope in the substrate. Even then, with better hardscaping techniques it isn't necessary. When professionals transport scapes they drain all the water and then stuff the open space with packing material to keep things from shifting. It should hold the stones in place. Water shifting around can move the substrate so I would avoid that by ensuring its drained all the way. Maybe leave a little patch of open area that goes to the bottom you can siphon from when draining.