Hi!
Hitec equipment will run fine on either 4 or 5 cell battery packs. If you have long extension leads, say longer than 12-15 in., for servos mounted in the tail or in the wings you may be better off with a 5 cell Rx battery.
The reason is that when the servos draw current there will be a voltage drop in the leads because of its resistance, meaining that the voltage delivered to the servo will be slightly less than the voltage supplied by the battery.
In practice I don't think it will matter much with a model such as the UCD-3D.
Since I suppose that you will be doing some hot-dogging with your new aircraft the 6 servos will be drawing quite a lot of current. If it is O.K. from a weight point of view I therefore suggest that you use the 4-cell 1200 mAh pach that you have already got.

/Red B.