Several of my friends are "RIFTS Junkies"...
In general, they look at 3 things when picking which classes to allow, in order:
- MDC per level, SDC per level, or HP only per level.
- Magic or not
- Massive skill list or not
Most classes in Palladium's system are HP only per level. Since taking HP damage can be debilitating (optional, but my friends all use it), this means toughness of characters with HP per level is only increasing resistance to death, not injury, and most characters within take the "full physical package" - all the personal SDC (pSDC) boosting skills they can.
HP per level with MDC armor and weapons are still HP based - and even with the full physical package, 2 MDC damage past armor is lethal to even the toughest. Such campaigns, including Robotech RPG play, often result in insta-death...
Those with SDC per level still get HP per level - there aren't that many that I've seen, but boy, they are tough. Often tough enough to take one or two MDC (remembering that SDC beings/structures take 100 SDC/HP per 1 MDC) at first level. They're big, buff, but killable. I've not actually seen the books with these, but I'll note that I've been told about them, and seen the character sheets...
A variation on this second group is the SDC xX model - Zentradi, from Robotech, for example – where pSDC and HP are generated normally, then multiplied. much tougher than normal SDC beings, often capable of taking multiple MDC, they have all the normal limitations once the HP take hits. They don't have climbing SDC, but they have HPx100, so 1d6x100 HP per level means being able to take 1d6 more MDC per level. With optional debilitation if the HP damage option is in use.
Those with MDC per level are another magnitude entirely - only MDC weapons hurt them. They often do MDC damage unarmed. The only things that really threaten them are other such beings, and people in mecha. Squishies need only be hit once by them, and crunchies (MDC armored SDC beings) usually don't survive the armor destruction.
So the secondary consideration is amount of magic/psi in the class. If they have a lot of ISP or PPE, they're more buff than their type of damage capacity might indicate. A mind mage can neutralize an enemy with MDC... even though he's a HP/Level being. In a group, high magic characters of a lower damage capacity type can "hold their own" by careful tactics.
The tertiary consideration is skill lists. Certain classes have wide and flexible skill lists. This makes them suitable for higher level campaigns than their magic and damage capacity might indicate, at least if the other party members are willing to shelter them from direct harm.
But remember also: good party dynamics can overcome the diversity, and many MDC classes become SDC=MDCx(10 to 100) after crossing some rifts.
As an aside: For me, the power creep was just too much. I quit Palladium almost entirely shortly after Rifts was released. I'll run Mechanoids.