I think those "beetle things" you are talking about are common pond snails.
Hitched a ride in with your plants.
Some people love 'em, some people do not.
Either way, hard to get rid of with shrimp, and not many shrimp-friendly fish that I would trust, either. The tetras will do more damage to them than snails will.
There IS in fact a scavenger beetle that you do NOT want in your tank.
It's quite large, rare in aquariums, and no mistaking it.
There are also scuds and water fleas, and people have a love/hate relationship with those, too.
As for your algae, and your sword(?) plant deterioration, I think posting your water parameters would be helpful for diagnoses, and the more plant you have in an aquarium; the better