If rock were metal and either ghost or poison moved to darkness, then each TCG type would have 2 VG types per type. If only Fire and Lightning were combined... but I do like it split though.
Colorless can have 3, because it can use any energy, while Dragon can have one VG type, because it uses multiple energy types.
signofzeta, cards can use any Energy the designers want to have them use; we have had Colorless Pokémon (and not just when Dragons were a part of that Type) that used specific Energy Types in the past. Pokémon Type
usually matches Energy used, but often enough doesn't. While I certainly don't want :fire: and :lightning: fused into a single Type, if it was the Energy wouldn't be a problem for too long - new cards could use a new "unified" Energy (or only one of the former) and eventually the older cards would rotate out... well ignoring that we are happily getting a separate BW-On format for long-term play.
What is inherent to Pokémon-Types is Weakness/Resistance... to a degree. When we have 1:1 ratio of video game to TCG Type, the only discrepancies come from the changes in game mechanics between video game and TCG, such as the video games featuring multiple Weaknesses/Resistances (when present) and how Resistance is either half damage or zero damage (before stacking from multiple Types). When two video game Types are combined into one for the TCG, anything with Weakness/Resistance to one of the video game Types has to have the same relationship with the other, whether it did in the source material or not.
You can create a :colorless: Pokémon that is :lightning: Weak and :fighting: Resistant to represent the video game Flying-Type, but without complicated new mechanics or card specific attacks/Abilities, you've got no way for the Flying-Type to recognize "Rock-Type" attacks and take double damage... which matters because Flying-Types are
Weak to Rock-Type attacks while they are
Resistant to actual Fighting and
Immune to Ground-Type! You also have the same problem when a "Flying" :colorless: Type Pokémon is attacking - the other half of :colorless: is the Normal-Type which does half damage to Rock and Steel-Types and no damage to Ghost-Types. The Flying-Type on the other hand is supposed to hit Fighting, Bug, and Grass for double damage, Ghost for Normal, and Electric for half-damage.
I love the idea of Poison being combined in Darkness; makes a LOT more sense than being Psychic if you ask me. Rock with Metal I like a little less because Rock and Ground are so closely related, but I do admit that if we could only change one type to combine with Metal, Rock would make the most sense.
Yeah, this brings up the other aspect of adapting the Types; preconceived relationships. Even if two Types match-up fairly well, they need to make sense "together". Poison is odd since frankly, it doesn't make much sense as an "elemental type" but is just an agent for delivering an effect (at least in the real world). Still, for Pokémon video games it is a Type so we have to look at what suits its "flavor" as well as what causes the least amount of deviation; where combining the two Types together results in Weakness/Resistance/Neutrality to a Type that should be one of the other two relationships.
Before factoring in how the combined Types work together (that is where things get
complicated 
), just looking at the combination of video game Types versus the other non-combined Types, Poison matches up best with Darkness and Rock with Steel, though it is only slightly better than some other pairings. Fortunately they also make a reasonable amount of sense together (black being a decent coloration for poison and being associated with the kind of tactics favored by "Darkness", while Rock and Metal are at least both minerals and closely associated with earth) and
the big gain is that there is less deviation overall than when they were in their current relationships.
TL;DR: ...probably should just ask to see the spreadsheet I was talking about. :lol:
