I've grown out lents in a 55gal before, but that was with a lot of hiding spots, dithers, and a watchful eye. Even though they were only 4", they were still showing lots a fair amount of aggression. I took a chance, and it paid off, but it was very risky, and didn't last for long.
You have to understand that Crenicichla have large territories, and they defend them from other pikes with a dedication that is incredible. The fact that your venny is already established in that tank increases the potential for a brutal clash between them. You're probably going to get a shredded fish, and most likely a dead one in the end.
If you are SERIOUS about keeping some lents in the 125gal, I suggest you get rid of all your large fish, including the venny. Get a group of four to five of the lents and grow them out in the 125gal. They will grow quick and their aggression will continue to get worse, but hopefully you will be able to get a pair out of them. IF this happens, you'll definitely have to remove the other lents. EVEN IF you do get a pair, you will HAVE to keep an eye on them, because they can turn on each other unexpectedly, although its not as 'common' once they have paired up.
I've got eight pikes in my 300gal, and as long as everybody stays in their 'respective areas' there isn't too much aggression. The lent pair though has the largest territory in the tank, and are definitely the most aggressive pikes in there, and there are some bruisers in there. Food for thought.