When using a composite character like this, you only need to recruit the main form. If you make a shapeshifter and a temporary form, then only recruit the shapeshifter. You can train the temporary form at your own discretion by adding new poers to their hero file as the campaign progresses, but this information is not saved in the save games.
Recruiting a temporary form is a waste of prestige, and training them will not have any effect.
New Rumble Room modes are available thanks to M25's ceaseless efforts on the AI front and on EZ Script. In addition, there are also a few modes which actually constitute tools for preparing characters, etc.
In FFX2 and earlier, there was a system for bringing shapeshifters and temporary forms into the Danger Room together to form a temporary association. You no longer need to do this.
Only take the main forms, ie the shapeshifter or the summoner, and the temporary forms/minions will appear when you morph, or summon them etc.
Not yet tested with Freedom Force vs. The 3rd Reich. Attributes and power swaps may behave unpredictably in MP games - updating FFX 3 for multiplayer is not currently on the to do list. But hey, if you want to take care of making it work, you're more than welcome!