+1 on what Gary says.it would be fine for size because they don't swim much. But large plecos are aggressive and also they are very high waste producers. That pleco would be more than all the bioload you currently have in that. Make sure you are ready for that before you take it.
The number/size of the filter will only manage the ammonia, you're still going to have to do large water changes to remove the nitrates.By aggressive do you mean it might eat the rest of my small fish?
And would it still be maxing out my bioload if I ran two HOB filters, one rated for a 70g tank and the other rated for a 50g tank?