Hello there !
I still own a couple of old kabalites (first box ever bought), but in my opinion they are far to different to be mixed with the new ones : not the same armors, not the same weapons, etc. etc.
For conversion purposes, whyches weapons parts, and also skaven bits (trophies, etc.) can work great.
I use the old models as Trueborn (elite version of warriors) as in the lore they should be older, and therefore have older armors and weapons. I added some gold to spikes and armor and they elite role is clear
If you want a simple advice to blend them in your army and still have a subtle 'personnality' difference, you can paint their eyes in a diffrent color (for example : all kabalite warriors (new figs) with red glowy eyes, all trueborn (old figs) with blue or green eyes. Green works great with gold spikes)
Don't hesitate to add a flag (you got some from raiders & venom) to help them blend in the army.
Here's some Work In Progress pictures i got from my FB account (sorry for bad quality) if you want to inspire.
I don't have the pictures right now, I'll add a couple of fully painted trueborns & kabalites this evening
Frenchie, long time painter, trying to get in the 8th edition
, still converting & painting hordes of dark eldars Project log
Dark eldars ~4k points /
Battle sisters ~2k points