Because common names like thread fin, and pearl are fairly nebulus and can be different depending on you geographic location is, its hard to answer your post.
To me, thread fin means Acarichtys heckelli, which is a fairly large warm water species from northern South America (preferring temps in the 80s), which in the end needs 100+gal tanks to feel comfortable.
Pearl means Geophagus braziliensus to me, and is a cooler water species from southern Brazil, that prefers water temps in the lower 70s, and when put in higher temp water can become hyper aggressive. The braziliensus groups is also more of a hard water type, compared to Acarichthys.
I have lkept both, but I would never keep them in the same tank because of temp preferences.
Both will eat neons, although the braziliensus rather sooner, than later.
A heckelli
and below from the braziliensus complex Geophagus iporanguesus