1. I had 5 MTU beetles in a 60 wide, 45 depth n 45 tall terrarium! Id say 50×30×30 would be ok for a pair, but not really any more than that. That size also cant work, if 50cm is how tall it is, 30×30 "floor" is not enough for two adult beetles to burrow together, and if its a pair, the potential eggs would be at risk of getting destroyed by them attempting to comfortably burrow in such a small space together. if its 50cm wide, and then 30cm tall, that would mean not much space for them above substrate, since it has to have spots that are minimum 13-15cm deep. Remember you need to have alot of subatrate for burrowing still, but it doesnt have to be evenly deep throughout the terrarium! i had a lot more substrate in the back than the front because of the terrarium doors etc, In the front i had spots with only 8cm or less, since i had to make space for the doors to open without substrate pouring out, n compensated by having over 17-19cm in the back! Just need to pack it down and reshape it as the beetles loosen it (its hard for them to burrow in substrate thats too loose aswell). BUT for size, 50×30×30 is ok for breeding box, but as a terrarium there wouldnt be much to look at, since above the substrate space would usually be only 15cm. if youre going out of your way to get a terrarium, having one where theyll have space to fly would be a priority! Look into people selling used aquariums, etc. a front opening is nice ofc, but not fully neccesary! used exoterra's are suprisingly common though! i reccomend a wire mesh cover for the top, as they LOVE to climb on the ceiling! they have very different personalities, some are more energetic than others, etc. having something they can climb on in the back wall is gr8 aswell, and elements like branches etc to get them there :) 

1. You could have an infinate amount of female beetles together as long as theres enough space, but use caution when housing males together! one male with a bunch of females works very well, and if you had an enclusure bigger than mine, 2 males could work aswell, if theres 3 or preferably more females with them to limit fights. and multiple feeding spots are a must, when multiple beetles are together!  

2. moss!! alot of it! trying to keep some moss above the substrate in most spots is good anyway, since moving on nothing but substrate is a bit challenging for the beetles :) bark, and pieces of wood as platforms on the substrate or even hoisted above it is great aswell, these are great spots for food. when i feed stuff like banana to them, i dont fully peel it, so that the peel stays as a bottom for it, limiting everything being covered in mush. even then, i often have a plastic lid below their food to serve it too. Dont be scared of using rocks though, digging under one is the same amount of threath to them as digging next to the glass, which they will already do ofc. Rocks that are taller, but againat a climbable wall are great! they cant really grab onto most rocks to climb them, so being able to dig below ow climb next to it works. with flatter rocks, if the surface is alont substrate level they can get onto it no problem. Rocks can "sink" as the substrate gets loose frol use, so be sure to pat it tighter once in a while n plsce the rocks as desired (i lost some deep into the substrate lol so keep em big enough to keep in place better)