I vote Intv 1 styling (i.e. PlayCable-like).
As a 3rd option, you could model it after the Intellivoice instead of Intv 1 vs 2.
Regarding the slots, there are multiple cartridge molds out there. Some might not dictate what you can/can't release.
If you do decide to use the same mold for the ColecoVision SGM as the IntelliXpander cartridges, please choose a edge-card connector that is deliberately incompatible between the two platforms. The last thing users (and yourself) wants to deal with shorting out and damaging hardware because someone plugged the wrong cartridge into the wrong platform. Yes, you could print "for ColecoVisioon" or "for Intellivision" on the cartridge labels but we all know that labels get damaged or fall off or just get ignored. Think of this as a way to not have to deal with free replacements because someone's IntelliXpander "magically stopped working" (when they actually shorted it out).