No worries, in my 1st cube it was such a hassle trying to do work inside the small cube stand. For now, u can try tank as is and see if it's livable and discover any other items u'd want to improve. After few months, if u find it too cumbersome, then can always go w the 20 long.
When I upgraded to a 93 cube, I placed the tank in basement by furnace room and quickly got tired of having sump inside the stand. So I drilled a hole through the wall & placed a bigger diy sump behind the wall. No more tight spaces when doing sump work, now while working I sit on a small kiddie chair from the days when my boys were toddlers .. so much more comfortable on me old back!