I also use wii gun with Mame, Up to 5.20 for the moment.
Concerning Area51, better to use the combined Rom Area 51/Maximum Force Duo. You have 2 games together as it was in the arcade machine and Maximum Force it's somewhere even the best of the 2.
Now I would say that you dont have the right Rom already for Area51, because this games are the less complicated to put in place and work pretty good with Mame 078. Just you need area51mx.zip and area51mx.chd. If you look into the very legal archive.org/download/MAME_078, you will find.
One interesting alternate to Aliens 3 The Gun, is Spacegun from Taico. Unfortunately mirrored because the original arcade cabinet was working with a mirror. I have the right shader to put the image back reversed. So if somebody need.