@FiveGensOfLove - am only guessing here, as not had to do this set-up myself, but I’d say yes. And if you can have the pen fairly near to where you want the tray to eventually lice, then that might help.
A couple of things to bear in mind - and it won’t necessarily be easy with the pen! Cats do not want their food near the litter tray - keep food bowls as far away as possible. Also, they are not fans of food and water bowls near each other. Honestly, I’m making them sound worse than they are, but just bear in mind.
Long-term (ie when they’re old enough to have free reign of the house), general rule of thumb is one litter tray per cat, plus one extra.
Personally, I prefer a large tray that is covered - the thought of an open tray with them missing their aim makes me want to vomit! Plus a hooded tray gives them some privacy - especially if your home is fairly open plan (after all, who wants to take their daily ablutions when others are looking on?!)
With regards to litter, I always go with Cats Best. It’s a wood pellet clumping litter. It’s slightly more expensive than others, but it makes it very easy to clean out the “dirty stuff” whilst not wasting the clean stuff. You will get tracking, but really easy to clean up with a Hoover. And get a ridged mat to put at the entrance to the tray, it catches a lot of loose wood pellets.