Honestly, the cross string depends on the person. If it's a multi that we're talking about, then its the multi's characteristics that you'd need to look at. If you want power, cross with X-1, Xcel or Premier. If comfort, Mantis Comfort Syn or WC Explosiv! would work well. Simply playability, NRG2, Isospeed Professional or Maxim Touch would do. Personally, I don't find TNT2 to be particularly good at anything, so I don't carry it. Gamma Live Wire XP and Professional are excellent, though.
The poly mains vs. crosses is a long running debate on here. At the end of the day, it's preference. I hit with the B5E frame back to back with the gut/CoFocus frame. The difference is startling in feel, but the ball on the other side of the court is in most instances the same. It's only under certain situations where one would shine. The gut would suit the more offensive player while the counter-puncher or defensive player would likely prefer the poly. The all-court player can't make up his mind (aka, me!). You're basically either damping the liveliness of the gut with poly crosses, or enhancing the playability of the poly with gut crosses.