Yes, you can mix and match titleist irons anyway you would like. 

I was thinking about mixing and matching AP1 6-7 irons and 755 Forged 8-9-PW.   I can hit both pretty well; however, the AP1s have pretty much won out.

I picked up a set of 755s (3-P) that was missing the 4 and 7 irons.  I had the irons set as follows: 22-27-32-37-42-47.   My AP1 6-PW are 28-32-36-41-46.   The AP1s are 1/2" short and 1/2* upright with Excel RF grips.     The 755s are stock length, standard lie angle and cord grips.

The AP1 6 and 7 irons are a no brainer because I hit them the same distance as the 755 5 and 6 irons (lofts are the same).  The two PW are only 4 yards apart.   I was hitting my AP1 PW the same distance as the 755 9 iron and I was hitting the AP1 8 iron 10 yards farther than the 755 8 iron.

Without modifying anything, I would have a 20 yard gap between the AP1 7 iron and the 755 8 iron or the same gap between the AP1 8 iron and the 755 9 iron. 

I played a round a couple weeks ago with the 755 8-9-PW and the AP1 6-8,PW.  Did OK and the 755s actually worked better around the greens but lack of distance on the full shots is the main issue (barely hitting the PW 100 yards or hitting the 8 iron 120 yards; I hit my AP1 8 iron 130 yards and the PW 105 yards).

I may think about shortening the 755 PW to 35.5" and matching the lie angle to the AP1 PW.  It is a thought.

Yes, combo iron sets are the way to go because you have forgiveness in your long irons, and workability in your short irons.

I attempted AP1 7-8 and 755 Forged 9-PW along with my Vokey SM4 52-12 and 60-10.   They hit ok but the biggest thing is there is a 20 yard gap between the 8 iron and the 9 iron.  The 7 iron was a no brainer because it and the 755 6 iron (both 32* loft) get the same distance.  There was a 10 yard distance difference between the AP1 8 iron and 755 8 iron (AP1 36* and 755 37*).  The AP1 PW got the same distance as the 755 9 iron (AP1 36* and 755 9 iron 42*).  The 755 PW was only 4 yards less than the AP1.   Also, I found I can hit a 25 yard bunker shot in soft sand very well with the AP1.

With AP2s and CBs it may be a different situation but this should work.

The main thing about the CBs, most likely, is that it is easier to pick the ball clean off the turf and the AP2s might fare better off grassy lies.  This is my guess.