The GT coils fit the ProCore. I’ve used a couple. I haven’t tried the GT CCell because I don’t want to vape Ceramic.
The Baby Beast RBA doesn’t fit the ProCore tanks. It comes with an extended glass that makes it work on the Baby Beast tank but it’stoo tall for the ProCore.
Check out this Reddit thread. It lists a bunch of tanks and coils that are interchangeable.
I just bought a Vaporesso GT cCELL 0.5ohm 15-40W Clapton Ceramic coil to test out, I pre-fit it inside the SMOK Baby Beast and it fit fine, I’ll try it out in a ProCore tank next week. Hoping for TC. I just got word we are babysitting though so it will be next week to try it out.
I’ll be the test animal next week!
Of course my local shop don’t carry them so I couldn’t check one out for TC mode 
Installed said coil in ProCore Air on Espion Solo today (5/12/18), fit is great, no leaking (so far) and the taste is good!
And it runs in TC SS mode! Even though I like it better in straight Wattage mode.
The Wismec coils look like they might fit also?
Wismec GNOME King coils (need verification of fit):
WM-M 0.15ohm 40-100WBest 65-75W
WM01 Single 0.4ohm 40-70W Best: 50-60W
WM02 Dual 0.15ohm 30-70W Best: 45-60W
WM03 Triple 0.2ohm 40-130W Best: 70-90W
Medusa TFV8 BABY Staple Clapton Coils
0.25ohm Staple Clapton Coil 35-80W Best: 55-65W 