I'd say maybe. Picasso triggers are way more aggressive than niger triggers. If the niger is in the tank first and established than good chance they'll be fine. Especially with the size difference. However size wise no they're not even close to adults. Niger adults can reach 18" Picasso triggers top out about 15" I believe.
These are not super aggressive triggers that need species only tanks as they grow - they are not bluelines of queens. They are not even so-so tankmates like a clown trigger. They are easier going than these, IMO. I have kept these two together and would again.
If their needs are met with enough food to be growing, lots of real hiding spaces (not caves and arches that humans like, but just piled up rock where they can disappear) and decent water quality, they they should be fine. If they start to get aggressive towards each other, then they likely need more food, even if you think that you are feeding them enough.
They will eventually outgrow a 125g, so you need to have a plan that is best for them... like for free to a better home not worrying about the cost, price or anything besides their well being. When people do this only for the fish, I believe that they can be good stewards of large fish for a while. When people try and sell fish to make a buck and worry about themselves more than the fish, then this is where keeping fish that get large gets a bad name... but it does not have to be like this.