I would say that a pacu is a no-go. Gars are pretty prone to being bullied and out competed for food and I think a pacu would do the latter if not both.
I'm assuming you want to put it a pacu with your longnose? Personally, I dont think it's worth the risk. Longnoses already break their backs easy enough all by themselves, not to mention that Pacus get freakin' huge and last time I checked you had a 55 gallon (ignore this if you happen to have a pond or larger tank).
I'm sure you already know this...but be ready to upgrade that tank sometime in the near future as the osseus species are especially fast growers under good care.
this is where i contradict anything ive said in the past...although i strongly advise against this i recently came across a large pacu in desperate need of a new home. so....i have a 21+ inch pacu w/ four of my gar (absolutely no problems thus far...she is very gentle). feeding is the only issue, but i just wait until the gar are at the surface and drop the shimp right next to there snout. it seems to be working fine. again i dont recommend this, but it CAN work...it always depends on the individual fish's personality.
also...you have a longnose gar. they are one of the most easily injured gar...that makes it an even worse idea. plus i think ive read that you have a 55g tank...i would focus on your next tank rather than your next tankmate. just my two cents.