I have never tried this in GURPs, but I have played other games where there were large discrepancies in the starting powers of the characters. I have tried mixed old World of Darkness games with different species and different versions of DnD with widely separate starting powers (in fact, this almost happens naturally in later versions of DnD if you have skilled optimizers next to non-optimizers) and a few others.
Generally, it has been my experience that widely different power levels do not work well together. Someone will end up feeling overshadowed or someone will end up carrying most of the load, and it makes the campaign very hard to design for from the GM's perspective.
I know of two exceptions where it works quite well though.
For a new player, especially if they are a guest.
If you have an existing campaign going and you are bringing in a new player, especially someone who is new not just to the campaign but to roleplaying in general, then it can make sense to make them somewhat more powerful than the others. This makes it easier for them to have a chance to shine and it makes things more "fault tolerant" since the character has enough power to survive novice player mistakes.
I'm not actually recommending this as it can cause problems down the road as that player becomes more comfortable with role playing but still has the strongest character. But it can help introduce someone to roleplaying in a positive way, especially if that person is a guest that won't be able to play through the whole campaign or if it is meant to be a short campaign anyway. I've been on the receiving end of this when I was much younger and a new player and it was quite pleasant for me.
If someone wants to play a weaker character for narrative reasons.
I like gishy characters (fighter/mages). I will take a gishy character even if I know full well that it is suboptimal, as long as its not so suboptimal that I expect it to cause real problems in the power level.
In World of Darkness, the character types are not all made equal. Mages for instance tend to be more powerful than vampires and everything is more powerful than ghouls. But some people will knowingly take a weaker character type because they like it better. As long as the power discrepancy isn't so big that it will cause problems and as long as the player realizes what they are doing, this can work very well and keep everyone happy.
In general
In general, I really think the power levels should be close to equal. There are some exceptions, but those exceptions work best when the differences are small rather than huge and when everyone at the table understands what is being done and agrees to it.
I haven't played GURPS, but that looks like a pretty big power spread and I'm not sure how well it would work.
Your first stop for maps of Rifts Earth is the Rifts Game Master Guide. Pages 332 through 352 cover the borders of most notable locations in Rifts Earth: the Coalition States, Federation of Magic, Manistique, Northern Gun, Lazlo, Tolkeen, the Canadian Hivelands, the New West Territories, the Vampire Kingdoms, the Pecos Empire, Atlantis, the British Isles, Western Europe, Russia, Japan, Australia, and the Phoenix Empire in Africa.
The borders (current and hoped-for) of the Coalition States are also shown and discussed on pages 13–14 of Rifts Sourcebook.
Siege on Tolkeen: Chapter One describes on pages 112–114 the extent of the Kingdom of Tolkeen, and features a nicely detailed map of Rifts Wisconsin. Detailed information on European, Japanese, and Australian territories also appear in the last installment of this series, Siege on Tolkeen: Aftermath (pages 183, 188, and 196 for maps and surrounding descriptions).
Best Answer
Several of my friends are "RIFTS Junkies"...
In general, they look at 3 things when picking which classes to allow, in order:
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.